Probably the best advice I can give you:
Be sure they like you back before asking them out. (first guy I asked out I don't think I was 100% sure, but I had convinced myself because I wanted him to like me back so badly. Of course, I was rejected) If you can't figure this out, there are ways to find out, such as asking other people. Do your detective work first!
Persistence pays off.
As for actually asking them out, if it's more like a first date - you don't know each other really well but you want to get to know each other better, make sure it's really casual and friendly so you don't scare them off.
If you've spent ages hanging out, and you're both clearly infatuated with each other, and you want them to be your gf, then find a time when you're alone together, gather up your courage, and just ask them.
If you get rejected, don't let it destroy your confidence in the future.
I think (from your age) you're at school, so maybe don't focus too much on the whole dating thing (like in the first example). Go for the friends first, then asking them to be your gf.
Btw, well done, 15 is very young for an aspie to enter the dating world. You've done very well to get so far so quickly. I didn't have my first kiss and first bf till I was 19 (although I made an unsuccessful attempt when I was 18).
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Into the dark...