Why do I suck so much at black ops?

Okay so firs off idk what's the best gun to use I play on both ps3 and 360 ps3's mine 360's are my brothers anyways I sometimes use the ak47 with suppresor or the Galil with suppresor or famas with red dot but everytime I shoot at someone with the same exact gun as me it doesn't matter how many bullets I put in them they kill me help.