you've tried other FPS? let me guess Modern Warfare, World at War, Modern Warfare 2, Black Ops.
Because if you 'have' played other FPS, you'd notice COD games are the only multiplayer user friendly games online. Other FPS require team or skills: Unreal Tournament, Battlefield, Team Fortess, Killzone, F.E.A.R, Quake.
Why are you dying? because you are not reading simple instruction put in the game, there are video tutorials teaching users how to play Brink.
Are you aiming at the feet or body or head? If you do play many FPS, you should know that shooting the feet/legs does less damage.
Are you camping? You have been given a Sprint innovative system, why are you hiding and not using it?
So you want to kill rather than perform objective? run and gun is your speciality, then COD is written all over this.
I'm sorry I can't help you. If you read the tutorial it will teach you that Heavy are slow and carry heavy weapons, they can also take in amount of damages.
Medium characters move decent, and sprint better than heavy
Light character moves the fastest but dies quicker than medium.
why are you dying?