It would probably go along the lines of this:
Me: Hey.
Younger me: Who are you?
Me: I'm you from the future
Younger me: That's cool.
Me: Listen, you're going to fuck up multiple times in the future, I'm going to give you some precautions.
Younger me: Ok.
Me: First of all, stand up for yourself, and kick the shit out of anyone that bothers you. Second, in four years on St. Partick's day, do NOT drink any alcohol, don't even get the idea of drinking alcohol on that day. Third, teach yourself guitar and piano, as soon as possible. You got all that?
Younger me: Yes.
Me: good, never forget the advice I gave you just now, you will thank me for it. More like, you will thank yourself. See ya, or yeah, see myself. Whatever.
*disappears*
If I could go back in time, that would change my life drastically for the better.