I really like this girl and want to ask her out. We're best friends. Is that a no-no? How do I ask her out?

We've been best friends for about six months now. We've been friends for about three years but have really started to be really best friends in the last six months. I've always kind of liked her. But in the past three or so months, I've started to really like her and want to go out with her. I think she might like me but I'm not sure. How can I tell. And how do I ask her out. It would really help for a girls point of view on this. But anyone can answer. Btw we're both sixteen so it would help if someone the same age or older answered. No twelve year old know it alls please!!!!