Long post incoming, but if you truly wish to know about competitive Pokemon training, please read onward! But if you only wish to know which resource can help you because you're fine in finding everything else on you're own, look up these following websites listed in order of priority.
1. http://www.smogon.com/ as the competitive Pokedex analyzer for strong overused Pokemon, Pokemon tiers, and its online battle simulator
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.
End of intro - start of long post
Smogon is a website dedicated to analyzing over used Pokemon in the competitive battling scenery. In fact, some people say Game Freak works with them with each Pokemon game to make sure they are balancing the Pokemon metagame correctly. Smogon has bio for each Pokemon they bother to analyze but they will definitely analyze the strong overused Pokemon you may want to include in your teams.
They have details like
1. The Pokemon base overall stats
2. The Pokemon's popular move sets...
3. ... along with the Pokemon's popular IV and EV for said move set...
4 ... and even which held item should be equipped for that move set.
Here's an example of a Smogon entry for Alakazam, an overused Pokemon in Red and Blue to Black and White 2. http://www.smogon.com/bw/pokemon/alakazam
As you can see, they even have a a paragraph dedicated to check and counters to Alakazam if don't wish to have it in your team, but are afraid others have him and you wish to counter it.
Unfortunately, they are still working at analyzing every Pokemon again in the X and Y gen. But you can look at their X Y competitive metegame preview here: http://www.smogon.com/forums/threads/sixth-generation-pokemon-analyses-index.3494918/
Smogon also offers a FREE online simulator where you can a build team of Pokemon from Gen X and Y and customize each Pokemon's IV, EV, stats, movesets, and held items. Afterwards, you can battle other people who have done so as well. And there is sizable amount of people that do this so you won't have to wait for finding a battle. It's real time as well with a chat box and all that jazz. Do this first before you decide to train a certain Pokemon or team in your actual game. You cannot transport the Pokemon you made in the online simulator to your Pokemon game. This online battle simulation is called Pokemon Showdown! and I'll post the link below:
http://www.smogon.com/sim/
Serebii is also very helpful. It was the website that taught me what EV and IVs are and has a calculator that calculates your Pokemon IV. You cannot do this in game so this is an invaluable source.
Here's serebii's guide to EVs:
http://www.serebii.net/games/evs.shtml
It's a long read, but it gives you everything you need to know without cutting the corners. If you don't want to read most of it, then forget about going through training EVs. Knowledge is definitely power here.
Likewise, here's serebii's guide IV's.
http://www.serebii.net/games/ivs.shtml
It has a tiny section about X and Y below. But again, read this fully.
And if you even find a game mechanic term that is repetitively brought up in Smogon and Serebii, just google that term with Serebii, Smogon and bulbapedia attached to it.
Bulbapedia is Pokemon wikia page. And its massive and very helpful.
Disclaimer: I have never built a a full competitive Pokemon team and beat anyone else that actually had one. But I truly believe you should check the websites I listed above.
