I am 19. Here's my word-

1. If you develop a hobby at this stage, it would be with you forever, so go ahead and pick up one. I wish I started learning the guitar and swimming early on.
2. Studies are important, but what's more important is to choose what you want to do in your life(it's quite tough to predict it early on, but still, having a rough idea will do no harm). Don't follow the herd blindly. Try out new things if you are into academics. Trust me, your class 10 GPA doesn't help much in future, but your (academic) extra curricular skills will surely do. I feel I should have started Coding early on. I studied Physics at 15, but now I am pretty much interested in Computers.
3. Don't buy a mobile phone with internet.
4. Don't make an account on Facebook, Instagram, Snapchat, etc. Unfollow useless topics on Quora. (Life experiences and Dating & Relationships are filled with so much shit these days. And people are getting even more encouraged to write about their 'caring boyfriend' (-_-).
5. Going to malls and parties with friends and putting up cheesy posts on Instagram doesn't help in anyway, and a day will come when you'll be like-What the hell had I been doing all this time?' And don't try things which are not supposed to be done at your age.
6. Stay with good people. People who you see as 'above' you.
7. Don't run after relationships/girls/boys
8. Respect your parents. You will regret every time you did something wrong to them.
9. Don't come under peer pressure, stay who you are.
10. Don't end up spending too much time with your 'frnds fr lyf'. They will never meet you again after you enter class 11/college. Better improve yourself.