So I just got Steam and wanted to know what are ze "musthaves" so to say.

So list away plz. (I'll look into them as long as they aren't shooters)

Check out Valve games. Half Life's, Portal 2, TF2, Left4Dead's. Not sure what kind of games you like. Better to buy games on big sales so just grab one for now.

Edit: No shooters. Disregard everything. 

Civilization V
Dragon Age: Origins
Assassin's Creed Brotherhood
Europa Universalis IV
Deus Ex: Human Revolution
Mass Effect
XCOM Enemy Unknown
Assassin's Creed II

Mass Effect 2

Do you consider mass effect a shooter?

Civilisation V for turn based strategy.
Portal 2 for a Puzzle FPS.
Sins of a Solar Empire for an RTS that will suck away your life.
Sanctum and Defense Grid for Tower Defense.
The Settlers 7
Heroes of Might and Magic
Dota 2
Titan Quest

However, now you have the Steams, keep an eye out on humble bundles you can buy steam keys from for as little as $1.
www.humblebundle.com

And of course, keep an eye out on sales, the next one should be the Steam Autumn Sale, your bank accounts will more than likely cry tears of blood when the sale comes around. :P

starcraft 2!

Anything by Valve, Super Meat Boy, Psychonauts, Payday 2, Torchlight II, Rogue Legacy and Red Orchestra 2/Rising Storm, games by Paradox Interactive, Total War Series, Papers, Please, Cave Story+, Amnesia  to name a few.

Definitely have a look at some of Valve games! Portal 2 is amazing and other than being in first person isn't really a shooter at all, just a puzzle game with a stunningly told story. You might also want to give TF2 a try, it is a shooter but a very unique one (lots of different characters for different styles). it's also free, so no harm trying :P

If you haven't already played it yet, try out Skyrim. Having access to all the mods really makes it the best version of the game by a mile.

If you like strategy Civ 5 is a brilliant way to waist several hours/decades of your life. Starcraft 2 is also fantastic, but it's not available on steam (battle.net is still great for what it does though). The Total War games are also very good ^^

Also try out Amnesia for a great puzzle game :) It's very calming to play if you have a pants shitting fetish

The Binding of Isaac, Civilization IV, Torchlight II, Spacechem. That's a good start.

Don't know about must haves, but here are some of my favorites:

http://store.steampowered.com/app/8930 (Civilization V, latest iteration of one of the most brilliant game ideas ever)
http://store.steampowered.com/app/32360 (The Secret of Monkey Island, even today MI is still the best and most funny adventure IP ever)
http://store.steampowered.com/app/20920/ (The Witcher 2, great western action RPG)
http://store.steampowered.com/app/206440 (To the Moon, visual novel which will bring you to tears)
http://store.steampowered.com/app/212680 (FTL: Faster Than Light, great indie real time strategy/roguelike game)
http://store.steampowered.com/app/12900 (AudioSurf, brilliant music game which is played with the music tracks on your PC)
http://store.steampowered.com/app/207690 (Botanicula, very charming and unique adventure game without any spoken or written language)
http://store.steampowered.com/app/48240 (Anno 2070, arguably the best economic/building sim currently available)
http://store.steampowered.com/app/215160 (The Book of Unwritten Tales, one of the best "modern" adventure games)
http://store.steampowered.com/app/209950/ (Triple Town, charming puzzle game)
http://store.steampowered.com/app/38600 (Fairie Solitaire, good solitaire game)

Plus: http://store.steampowered.com/search/?snr=1_5_9__12&term=ys
(I haven't played Ys games on PC yet - I played them on PSP - but this classic series is always recommendable for JRPG fans)

Add VGC members to your Steam friend list and you can see what we own and play. I'm okr on Steam as well.

I don't think PC has any must haves anymore thanks to MS bringing the PC devs over to the console realm. Blizzard is even looking like they want in on the console action now.

Try the major RTS games like Civilzation and StarCraft. Valve also has the old classics like Counter-Strike and Battlefield even though on consoles now plays better on the PC. It will be just as good on the PS4 and Xbone next gen though.