Girls...help/advice please?

Ive been friends wit this girl from another school for a little over a year now and i started to really like her, only I havent told her yet. Im planning to take her to my senior prom this year- should i tell her that i like her when i ask her to prom or at the prom itself? and what is a way to tell her that she will think is cute and say awww or something?