I still remember a girl asking me for my number, I didn't give it to her because I felt it was a joke, in retrospect I wish I had to only see what would have come of it. I think I would also tell myself to be less afraid of talking to crushes etc. I'd also be a little more social outside of school.
Edit: I'd also learn how to study as think it would have served me more in college than high school, as it was a rude awakening learning how to study when in college as almost got put on academic suspension after two semesters and of course hurt my gpa etc.