How to let a girl know I'm interested?

This girl I like, we are friends and get on well. I get nervous when talking about things like this to girls. I mean REALLY nervous. I have had a million opportunities but I always chicken out. I can talk to her like a friend but I need to let her know I'm interested subtly. I think she fancies me but I'm not sure :S Can anyone tell me how to either subtly flirt or how to gain confidence when talking to girls? Just so you know I am between 12 and 16 years I will not say exactly how old.