Not an exhaustive list, but 10 things I wish I could tell my 15 year-old self.
1) Make space in your schedule to spend time with your family, especially your grandparents
At 15 years old you're time-rich. As you grow older your pot of available time gets smaller. So take time NOW, while you can, to hang out with your family; they won't all always be so near, and some of them won't be there at all. (And yes, grandparents can sometimes seem 'boring' and 'not with it', but they have huge amounts of life to share and wisdom to pass on. You'll thank yourself later for spending time with them now.)
2) If offered the opportunity, any opportunity, to go abroad while you're young, take it, no matter what it means you'll be missing at home. Travel will change you for the better in ways that you cannot comprehend right now.
3) Learn, master and practice the art of remembering people's names
It feels like a complement when someone you've only met once remembers your name the next time you meet, like "hey, I must have made a good impression if XYZ remembers my name, we only met briefly before". Practice (it really is a skill) learning how to remember someone's name and people will appreciate you for it.
4) Confidence is the best asset you can possess and a fragile thing to maintain. Used correctly it will open doors and help you in ways no amount of education can. However, beware the thin and undefined line between confidence and arrogance.
5) At some point you will have to let someone down who, for whatever reason, looks up to you.
How you go about doing this will define you in their eyes for a long time. So, before you do it, take some time to think about how it's going to be from their point of view and adjust your actions accordingly, but don't, don't let this reflection paralyze your action.
6) No amount of time spent in front of the mirror will make you more attractive
Fretting in the mirror is a waste of time. Girls are way more advanced than to base whether they like you or not on how you look - accept this and your life will be easier. There's really very little you can do to exert influence in this sphere despite what you might think or be told. Fashionable hairstyles, in particular, should be avoided at all costs. They usually only suit a lucky few and desperate attempts to copy them will only serve to make you look foolish in photographs you'll look at in the future.
Hard-setting, wet-look gel should never be used. Period.
7) Realise, truly, truly understand that you are likely to spend more time working a job than anything else you do in life. Be hyper-critical in how you spend this time - you don't get it back.
8) Taking yourself too seriously is unattractive.
9) Don't ever stop asking obvious questions
For some reason, as people grow up they stop asking the IT'S-STARING-YOU-IN-THE-FACE-IT'S-SO-OBVIOUS questions. Maybe it's from fear of looking stupid. Maybe it's just laziness. Maybe it's fear of confrontation or change. Whatever. What you should understand is that some of the most brilliant and successful people out there are exactly that just because they continue to ask the obvious questions. Keep asking them yourself and you'll be that little bit closer to being brilliant.
10) Posture is important
It just is. Everyone looks better with good posture. And everyone wants to look better, right?