I chicken out every time I'm with her?

I really like this girl and I am pretty sure she likes me. Even if she doesn't our friendship is strong enough that asking her out will not ruin anything. Well I really want to tell her how I feel but I always chicken out. Well tonight I had it all planned out. We went to the movies and I was going to write a cute note and put it in the popcorn or something. Well we didn't get any popcorn so there went that idea. I really want to tell her. Should I just text her telling her how I feel and the story behind me falling for her? Would this be to corny or what? She is 19 and I am 17. Please give me advice on some way to tell her like ASAP so I keep her interested. Please try to make it cute. Thanks.