Girls please help me out here what should i do?

ok this is the thing i'm 19 and i'm a very shy person and i get extremely nervous around girls i like and sometimes i cant even talk. and i am a funny guy i can speak up in cl with no problem and i do make a lot of jokes in cl too and i do have a lot of friends and very out going but not so good at expressing my feelings when i'm in love or sad only happy and anger. i really like this girl in cl i catch her staring at me all the time and looking away when i look at her and she eve smile at me out side cl but i ignore her, and i found out she has a BF they been together for about 1yr and is a senior in high school. so what should i do shall i go and talk to her or do the right thing and stay quiet and let it go i only have 2 more cles left. also my friend gave me her aim sn and told me to talk to her but it would be wired coz we never talked and i don't have the guts to do it. besides i'm a boxer i'm not used to being a sissy and spilling out my feelings lol