How to unlock scopes in MW2?

Hi, i'm super new to MW2 and I know you can unlock new scopes and perks and stuff. I use the barret 50 cal all the time and I keep getting these barret 50 call mastery things. I just want to know how I attatch the scope I earn on to my gun. Also, can someone explain to me what perks are? And what theyre used for? By the way i'm a Corporal II .