Well its a good start that you're already friends - that always bakes a great foundation for a relationship. About the other boyfriends thing - Id advise you not to ask her out when you know she's got another boyfriend. This will put her in a very awkward position, and she may think that a) you didn't know about the other boyfriend, and that you don't care about her enough to know these things; b) that you automatically think that you're better than her current boyfriend, which she may resent. So you might want to make sure she's free before you try anything.
About actually asking her out, I wouldn't freak too much. I mean if you guys hang out all the time anyway, it's obvious that she likes you enough to go out of her way to spend time with you. Also girls are easy - we're emotional and sentimental. If or when you're game enough to ask her out, give her a lot of praise, compliments and flatter her. Tell her you've liked her forever, pick out some specific things you love about her and tell her. It's probably best to do it face to face, and again she'll be flattered if you're embarrased about it.
Good luck, from Jackie