I'm a guy, so obviously a girl is going to be able to answer your question a bit better, but from what I hear, girls always describe an orgasm as first a build up, then an explosion of pleasure that leaves them feeling warm and fuzzy. (bloddy wierdos).
Anyway - As for how to reach one - Well, we're not allowed to give "how-to's" here, so we cant really tell you. Best help I can give is to communicate with your b/f. When he does something good, tell him. When somethings not so great, tell him. Eventually he'll work out exactly how to push your buttons. And you MIGHT then have an orgasm. But remember, not all girls can have an orgasm from fingering. Alot of girls need something such as clitoral stimulation to have an orgasm. So, you two are just going to have to work out how your body works.
[-]Better To Die On Your Feet, Then Live On Your Knees[-]