What should I do? Two girls are involved?

This girl I really like who I think likes me but I'm so shy around thinks I don't like her asked me if I liked this other girl Stacy while smiling and kicking my feet. I said I don't really know her and that was that. I went with some friends and stacy to a movie on saturday and the girl I like was supposed to go but then didn't go after she found out I was going. (She said "so you're going with stacy") Now stacy asked me to go to another movie, this time with her and a bf/gf couple, making it more one-on-one. The truth, is, I want the girl I really like, and I want to tell her how I feel. Do you think if I tell the girl I really like that I like her and she rejects me that she will tell Stacy about what I said? Or will I be able to go out with Stacy if it doesn't work out after I talk to the girl I really like? And should I go out with Stacy again or make an excuse not to go this time and wait until I talk to the girl I like so the girl I like can't use it against me...like "oh you like stacy"