If you've been friends with a girl for a long time, I seriously don't recommend trying to turn it into a relationship. In the worst case, you won't get the relationship and lose the friendship. I have a female friend (I'm male) that I've known for five years. We've had our ups and down, but were always there for each other. In fact, I wouldn't trade the friendship with her for anything. Instead of wishing you and that girl had a romantic relationship, try to see it from this perspective.
Long story short, relationships you've spent years building can fall apart in seconds. But friendships usually stay for years, if not for life, unless one or both people do something to destroy it.