I had a 19 year old girlfriend when I was your age, hb 8 bye the way. Not kidding
my advice would be that you behave like your the MAN with a capital M, be a complet alpha-male around her and around her friends. I know that it was that wich got me her in the first place, older girls really seem to like it when you dont give a damn about the age difference and act like an "eaqual".
oh, and be funny but dont tell any jokes that makes your usual circle of friends laugh no matter how funny you think they are, remember your 14 and South Park isnt as funny when your a bit older