About this girl hu i wanna ask to formal?

ok, this girl is a friend of ine i would like to ask to formal. i think she is gona say yes cos she didnt think she would go but then got the permission of her parents. and she said she was going and so i'm goin too. but i wanna ask her in a nice way so we could go as "dates" . so what ideas do u guys have on how i can ask her? secondly, i am kinda new on the subject of proms and stuff like that so i wanna have an idea of what happens