Ok, I'll get real with you and give you some real advice not all this macho BS these chumps are trying to feed you.
You're just a kid. As much as you're heard it before, I'll say it again. You shouldn't be having sex at your age. Could you imaging talking to this girl about an abortion before either of you can legally drive?
If you like this girl, don't even fake it. Get to know her. Talk to her. See if you like who she is. Do things together. Even if you're just friends now, if you stay with it, you'll be the guy she ends up with forever. The boyfriend is really not an issue. If you are there for her emotionally, she stay by your side forever.
However, if you just want to poke your meat stick in her because she's hot, don't. Get some prono and take care of that situation other ways.