Speaking (typing) as someone who has been 13 and as the father of 2 children (1 girl/1 boy) I can't say much that hasn't already been said... having said that however I would advise you to;
1. Dress for comfort rather than just the effect
2. Listen to the music that pleases you, not just the music dictated by your uniform
3. Do as you would be done by - this last is probably the most important. If you happen to get into a debate, remember it's probably not life and death, it's an opinion and if you point this out to your debating opponent leavened with a little humour, you may find yourself fitting in a little easier
I always wore plain blue jeans and a selection of t shirts. Back in my day we had mods, rockers, skins, new romantics, rude boys/girls, punks, new wavers, heavies etc. I like music from all of the above and more so why paint yourself into a corner by affiliating yourself exclusively?
Mostly though, have fun and play guitar at every available opportunity. I wish I had started earlier!
Good luck
x