The only thing I would do is urge myself the importance of chores and the like and to be better with brushing my teeth and eating healthy haha. That's pretty much it. Everything else happened the way it did to make me who I am. And that is why I am hesitant to even change anything, lest that instance be a pivotal moment in some obscure way.
Being wrong isn't "bad", failing to admit that you are, is.
Id tell myself to not be so scatterbrained and pay attention more, its gotten me in trouble in more than one occasion, Id also tell myself I need to work and try harder to get things and be more assertive.
I would tell myself to get involved in sports earlier because ages 12-14 were not good years for my body at all due to my lack of involvement in physical activity. The years after that were fine but that three year period was pretty miserable for me.
I would tell myself to be less of an attention-craving idiot who sabotages her relationships with people for her own enjoyment. I was not a cool person when I was immature on the internet, honestly.
I would also tell myself to buck up and do my homework when I was supposed to so future me could avoid the incident I recently had with my mother that was very violent, all over grades, too.
I'd first wear a disguise when I meet my past self, then I'll give him a Yu-Gi-Oh! deck...with a note saying "go to the school's cafeteria after school" then I'd reveal myself to be future him and duel him.
I just figured that it would be awesome to do that.
In the end I'll tell him, "never, let her go" I figured that that will freak him out.
Make sense? Oh, what fun is there in making sense?
Black FC: 1592-5761-9212 Pokemon X FC: 2921-9174-9000
One crucial piece of advice I'd give myself is to not solicit your email to a stranger. Cause doing so will make your life a living hell. I'd also tell my younger self to be brave and ask that girl out. Don't do what I did and let her get away from you.
The biggest warning I would give myself would be to start job hunting by my third year of college, so that I might actually have had an easier finding one at all. Holding off on looking for work has cost me dearly...