Step 2: Your role
If your good at running really fast for ages, you should become a scout, if you are great at aiming, you should become a sniper ect.
Try to buy a gun that suits your role, for example, a sniper needs a camouflaged sniper rifle, and a support gunner needs a machine gun to lay down a wall of plastic.
Also think of what system you like best, spring for one cock per shot, and gas or electric for automatic firing.
If your constantly changing role, just buy an assault rifle.