I'm pretty much a newbie in Pokemon. I tried first Pokemon with Black and White 2 but didn't play far, because of work stress and the announcement of X/Y. I completed Y now and are working my way to complete the Pokedex and get all TM. But as I tried a few times online battle, I could see I'm not in the least competitive. So I want to learn to build a competitive team. It have not to be the perfect team, only good enough to win some battles and lose others. So I need some tips, how to uild a better team. Remember I'm a complete newb, don't start with advanced stuff , I want work my way up from he basics.

All that is posted here onwards is shamelessly stolen  from all your great answers. Too much to give credit in detail, read the answers to get the full picture.

Useful resources

1. http://www.smogon.com/ as the competitive Pokedex analyzer for strong overused Pokemon, Pokemon tiers, and its online battle simulator http://www.smogon.com/sim/

2. http://serebii.net/games/mechanics.shtml from http://serebii.net/index2.shtml Pay attention to the right side bar that begins with in depth Mechanics, followed by Effort values and Individual Values

3. http://bulbapedia.bulbagarden.net/wiki/Main_Page the wikipedia page for everything related to Pokemon. Including the competitive meta game mechanics.

Starting point

Map out your team at paper (or whatever), taking into consideration what you expect from others and have stuff to counter it. Test it with http://www.smogon.com/sim/ to check weaknesses. Start breeding and training the team after that in the actual game.

Types

Types are very important. Each attack has an associated type, each Pokemon one or two types.

Type table for current gen X/Y (see also: http://www.serebii.net/xy/typechart.shtml ):

0 means, the attack deals no damage, + means the attack is effective and deals double damage, - means the attack is ineffective and deals only half damage. This is multiplied, if thePokemon has to types the attack is effective against, it deals four times the damages.

Attacks

The attack damage is influenced by the base power of the attack, the attack stat of the using pokemon and the level, modified by other stats like type. The full formula is found here: http://bulbapedia.bulbagarden.net/wiki/Damage

Here is the formula:

For physical attacks the normal Attack of the using Pokemon and the Defense of the defending Pokemon are used, for Special attacks this are Special Attack and Special Defense.

Modifiers:

So, I've read a bit about stats and IVs and EVs, but it seems like this is the second step before the first. I thought, the way to start is about thinking which attacks/attack-sets are good for a competetive team and then looking up the Pokemon who can learn these attacks.

Question 1: Is this right, is this a good way to start?

Question 2: Attacks have some stats, power and accuracy. Power probably affects damage and accuracy the probability of the attack to hit the enemy But obviously stats like Defense and attack also influence this. Is there some sort of explanation or a formula, to describe more in detail which effect each stat has on the result?

Question 3: Some attacks have the type special, I assume this means Special attack and special defense is used to calculate the result, am I right?

Question 3: Some attacks hit multiple rounds, like uproar. Is the power meant for each of the attacks or for all attacks added together? Same for attacks with multiple hits in one round, like fury attack.

Question 4: How much should I look after side-effects like poisoning, or should I go simply after max damage and accuracy?

I think the attack-set should be diverse enough, so that I could be effective or at least not inneffective against each of the Pokemon-types. Is this thinking right? After that I should choose Pokemon that canlearn these attacks and hopefully have the same type as the attack, to gain a bonus. I also think I should choose Pokemon with good base stats in the right category, so for moves in category special with high special attack. Is that right? Any other tips to start of developing a good team?

im a n00b too so i dont play online lol.

Yes, but I'm interested to play a bit online. Not that I expect to compete in tourneys. But enough to survive a few rounds would be nice.

when i eventually get around to finishing the game ill challenge you so you can beat me lol.

Well, challenge accepted! But don't be so sure I can beat you currently.

1. Well if you want to play online, and be successful, you're going to have to take EVs/IVs into consideration.

2. Attack Stat = physical move, Special Attack Stat = special move
Defense Stat = resistance to physical move, Special Defense State = resistance to special move
Speed Stat = The order of turns, faster Pokemon go first

3. Fury attack will hit two to five times in a row within 1 turn and has 15 power each time. So 15 x X amount of hits
Uproar will lock the Pokemon using it into the attack for 3 turns with 90 power each time.

4. Side benefits are nice, but are unreliable if they don't always occur. It's better to use a move like Toxic, which will always poison, over a move like Smog, which deals damage but will not always poison.

You will generally want to match physical moves with high attack stats and special moves with high special attack stats. You will want STAB (Same type attack bonus) moves, but also coverage moves. There are also priority moves, which will always go first, no matter the Pokemon's speed. If both Pokemon use priority moves then it goes to the higher speed stat.

Example...

Charizard

Fire Blast 120 power / 85 accuracy / 10% chance of burn (STAB)
Air Slash 75 power / 95 accuracy / 30% chance of flinch (STAB)
Hidden Power Grass 60 power / 100 accuracy (Coverage)
Focus Blast 12 power / 70 accuracy / 10% lower special defense (Coverage)

I'm not an expert (wait for Smeags, Tbone or Lestat to show up) but:

1 - Yes and no. While you can re-train EVs, natures and IVs do affect stats and cannot be changed. Good IVs can be trickier and time-consuming to get, but there's no excuse to waste time training a pokemon with a bad nature with the tools available nowadays *cough*breedingwithdittoholdingaeverstone*cough*.

2 - Yes. to all. This is the damage formula (quite pretty!) (found in this Bulbapedia page, more info there):

Of course there are plenty of modifiers:

3-1 - Yes.

3-2 - It's the damage per hit both cases. Every hit of Icicle Spear hits for 25 base damage, every hit of Outrage hits for 120 base damage.

4 - That depends on the pokemon. Some are good at playing defense, others offense.

Don't subestimate stat-boosting moves like Swords Dance or Dragon Dance. After 1-2 uses a pokemon with the right moves can be deadly.

As a rule of thumb, go for an attack-hindering nature for your special attackers, and a special attack-hindering nature for your physical attackers.

Oh, and Stealth Rock is overpowered.

Honestly, the best way to learn to battle competitively is to build standard team that someone else who's good at Pokemon uses, and literally practice by copying their strategy. You sort of learn the rules of battling more organically that way.

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Question 1: Is this right, is this a good way to start?

Not really. More important things to learn are Pokemon categories and types. Types are literally the most important thing about competitive battles. If you don't have a nearly flawless grasp on type match-ups, you will lose with even the best teams. By battle categories, I mean the role each Pokemon will play on your team. There are Walls, which are meant to be durable Pokemon that can outlast your opponent. These Pokemon usually have high defensive stats but low attacking stats. There are Sweepers, which are meant to be your power houses. They usually have high offensive and speed stats but lower defensive stats. There are a lot more, but in the beginning you want to just focus on these two. Who's good at attacking and who's good at defending. That balance can be key.

Question 2: Attacks have some stats, power and accuracy. Power probably affects damage and accuracy the probability of the attack to hit the enemy But obviously stats like Defense and attack also influence this. Is there some sort of explanation or a formula, to describe more in detail which effect each stat has on the result?

Yes. The damage that an attack does depends on those things and type. The higher the user's offensive stat, the more damage the attack does. Once that's factored in, if the attack you use is the same type as the Pokemon using it, the attack get's a 50% power boost. That's called a Same Type Attack Bonus. (STAB) After that, the higher the opponent's defensive stat, less damage the attack does. Lastly, depending how well or not the opponent's type resists the attack, the over all damage can do as much as 4x the damage or as little as .25 times the damage. The order I think is Base Power - Attack Stat - STAB - Defense Stat - Type Match Up.

Question 3: Some attacks have the type special, I assume this means Special attack and special defense is used to calculate the result, am I right?

Yes. Psysical attacks only incorporate the attack stat of the attacker and the defense stat of the opponent. Special attacks only incorporate the special attack stat of the attacker and the special defense stat of the opponent.

Question 3: Some attacks hit multiple rounds, like uproar. Is the power meant for each of the attacks or for all attacks added together? Same for attacks with multiple hits in one round, like fury attack.

In both cases, the power is meant for each individual attack. That's why attacks that hit multiple rounds are generally more powerful than multiple hit in one round moves.

Question 4: How much should I look after side-effects like poisoning, or should I go simply after max damage and accuracy?

Status effects are really important, but you need to learn more about both the statuses' effects and the types of Pokemon you want to use them on, and the types of Pokemon you want to use them. Burn lowers the physical attack stat of the opponent by 50%, so it's best to use on Pokemon with a high Attack stat. It's best to have a Pokemon with a good enough defensive stat using an attack that inflicts burn. Paralysis lowers the sped stat of the opponent by 50%, so it's best to use it on a Pokemon with a high speed stat. Again, you probably want a Wall inflicting this status. Regular poison is outclassed by every other status and should never be used in competitive play. Badly Poison (Only Toxic and Poison Fang can inflict this status) lowers the opponent's HP more and more longer they are on the field. This is good against sweepers (if burning and paralyzing are inaccessible options) and especially Walls. Walls and really fast Pokemon are usually best for inflicting the badly poison.

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I think the attack-set should be diverse enough, so that I could be effective or at least not inneffective against each of the Pokemon-types. Is this thinking right? After that I should choose Pokemon that canlearn these attacks and hopefully have the same type as the attack, to gain a bonus. I also think I should choose Pokemon with good base stats in the right category, so for moves in category special with high special attack. Is that right? Any other tips to start of developing a good team?

Eh. The golden rule is diversity. Never have two moves of the same type on one Pokemon. It's also good to learn types and type combinations with good coverage like Electric-Ice, Electric-Water, Dark-Fighting. Ground moves have good coverage. Rock moves have good coverage. Ice, Fire, Fighting have good coverage.

The most important thing to do is think of what kind of Pokemon you want to be the focal point of your team. What's it's role. What are it's strengths? What are its weaknesses? Add Pokemon to the team that compliments it's strengths and supplement it's weaknesses.