I am the wrong person to ask for this advice, but with a little googling and some advice from others, here are some tips to ask someone out to prom (and some things to avoid).
1: Ask them directly, not over a text, smile and say, ¨Will you go to prom with me?¨
2: Get them a small, thoughtful gift.
3: Make sure they are okay with it before doing something elaborate, like a sign.
Things NOT to do
1: Ask someone out in front of a crowd of people. This puts pressure on your date to accept and seems quite terrifying.
2: Use a cheesy or gross pick-up line.
3: No strange topics.
4: Don´t sound arrogant.
5: Absolutely no bringing up your date´s body; that comes off as harassment.