How do I tell this girl I like her? ?

There is this girl in my Homeroom cl that I really like. I am shy and I dont know what to talk about every time I try to talk to her. We have alot of things in common from as far as I can see and I think we could have a grate time together except I dont know how to tell her how I feel. and also I think she might like me because every now and than I catch her looking at me and when I turn my head she quickly looks away and when I talk to her, which does not happen often like I said, she kind of looks away and talks a little bit softer than usual