How can I tell her I like him?

Okay, well Im at 6th form which is basically college, and i've met this really nice lad called Kris, the only problem is that some other girl likes him, and I was stupid enough to tell her, that if she wants, she can have a try and go out with him. The only thing is that when I said it I never knew i would actually start to like him. Now I don't know how to tell her, that i want to go out with him! Any ideas?