Girls help? Does she like me and what should I do? (I'm shy.)?

There's this girl I like. We've been good friends for 2 years. She always tried to talk to me when we weren't friends (My sister is friends with her sister, and they constantly tease about us) Her friends all of a sudden became friends with me this year. In the yearbooks she writes things longer than most other guys. I'm really shy, like I don't really talk to people I'm unfamiliar with. I want to ask her out, but I don't know how and I'm afraid that if she doesn't like me back it will really screw up our friendship... Help please.