Play ME1 before you play ME2.
ME1 has many elements I prefer over ME2, but as khuutra said the 2nd improves in shooter/combat mechanics. At the same time though, ME2 loses some things I liked in ME1 such as more open environments and some micromanagement. Changing armour, guns, ammo types, etc was fun in ME1. It's a bit of a trade off, and there are some problems with each game, but I honestly prefer the first over the second.
Also download Alien Swarm on steam. It's a fun, free co-op arcadey shooter from valve.
Demon's Souls Official Thread | Currently playing: Left 4 Dead 2, LittleBigPlanet 2, Magicka