Go to a local shop and see if you can play a game using an army. Here is my take on the different armies:
SM and CSM: Good shooting, Armor, Close combat, everything. The best all around army IMO.
* Started a CSM army recently...still developing. *
Tyranids: Close combat and big bugs. You won't win any shooting battles, use cover, get close, provide lots of scary models to draw fire and hope to outlast them!
* I play bugs...win more than I lose. *
Tau - not as good a shot as SM...but have great guns. BAD in close combat.
Orks - Another close combat army or army that kills with shooting through mass fire power.
Necrons - robots, good guns, keep getting up after you shot them.
Eldar - fast, mobile, good units for clsoe combat, some good units for shooting.
Imperial Guard - grunts, same shooting as Tau but regular troops weapons stink. Guard shine with vehicles and ordance (BIG BOOm) weapons!