Just say hi, I'm not even kidding. You can just go up to her whenever and try and strike up a conversation so that if you ask her out or whatever it doesnt seem so awkward and out of the blue. You kinda have to just put all fear behind and just go for it though admittedly that's easier said than done.

As for conversation topics? Well where do you guys see each other? If she goes to your school you can ask her about what homework she has or how her day went or if she's doing anything this holidays. Maybe you have similar interests? You can talk about them too. And if you don't know what you have in common ask her about it! "What sports do you play" "Do you like music" etc. is so plain but seems to work.

You're only 15, even if she's the most beautiful girl lined up for miss universe the next 100 years take it easy and be friendly.

Oh and when she talks I have three words for you: Nod and smile.

like said, Say hi. That is a huge step it means you acknowledge her. And it means you don't think she is just another body in the group. Say hi and introduce yourself, ask how her day was. Make busy conversation. Talk about her life, your life, family, friends, jobs, colleges, relationships, siblings, past, future, etc Get to know her. Ask her to hang out sometime once you get to know her. And go from there.