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

 

"STAND BACK, THESE ARE WINTER BOOTS!"

 

You're golden, next time you hang out pull her aside and ask if she wants to go to dinner, watch a movie, mini golf (standard 15 yr old dates). Make sure you don't sit there awkwardly silent on the date tho, joke and make her laugh, easiest way for a girl to want a second date is to make it fun.

You already knows she likes you (hardest part), now just have some fun and get to know her a little more.

 

Asking her to be your girlfriend straight off the bat would be a bit weird