I would tell me to worry less about what others thought about me.
My sense of style at that age was less defined, or maybe I wasn't brave enough to wear the things that I liked. I would tell my younger self that probably everyone else is worrying about what they are wearing and they are too busy to be interested in what you are doing. So just do what you want to do, as long as it respectful and doesn't hurt others it really shouldn't be that big a deal.
Go see places, do things now. Don't think that you will have lots of time to do stuff. Time runs faster the older you get. The more commitments you have the less you have time or money to do things.
Spend time listening to the stories that grandparents and parents tell. Make notes. I regret not listening harder to stories, not taking notes and not committing to paper what i have now forgotten.
I was nice, I was never hurtful or spiteful at that age. However, I did get myself into a few sticky situations by not following my heart and conscience. I would say to have courage of my conviction.
But in general I am thankful for all the things that have happened. I have learnt a few lessons, but it might have been easier if I had listened better.