Even if she likes you, I guess it would be kinda creepy if you just ask "hey, wanna be my girlfriend?"
Are you her friend? Do you know her?
If not, you should befriend her - if she really has a crush on you, that won't be a problem. (Don't be afraid of the "Friendzone")
If you two are already get along pretty well, a simple "Hey, I have to go to [somewhere] this afternoon, it's kinda boring if I go alone, wanna come?" can be fine, too
(plazas are great, for example - we often go there with my brother together when one of us needs a new earphone, a video game or a book - and most plazas have cinemas!
you can ask her "spontaneously": "oh, hey, I heard that's a good film, wanna watch it sometime?")
Of course it depends on her (and your) personality, but hey, if I were her, and my crush asked me to watch a film with him, I would say YES immediately