I need help.......Should I tell her?

Well I like this girl from school and she likes me back. But she doesn't think I like her because I used to talk to her about this other girl I used to like named lona. But I just used to have a little crush on lonauntill I started talking to the girl I liked named valerie. I started to like her when me and her one night did this thing that they call an allnight err. then I started talking to her more at school and hanging around with her. Now I like her ALOT I just can't stop thinking about her and she has told me she has feeling for me. But she thinks I like lona but I don't. So valerie just broke up with her ex boyfriend because they fought alot and he was a dick she says. So I want to tell her how I feel before it's too late. Do you think that's a good idea? And what should I tell her? THANKS!!