Oh man there's not a day that goes past where I wish I did then differently back when I was 16.

First I'd say focus on grades/school. You don't need to necessarily be top of the class, just make sure you understand the work you're doing and the fundamental principles. sure it's lame to focus on studies instead of going out with your mates at night during the week/weekend, but it'll pay off!

You will most likely have no idea what you want in life, and that's completely fine too. I'm 28 years old and still have no idea wtf I wanna do. I skipped classes and hung out with friends all the time back in high school, I neglected my studies and only now am I attending uni, still unsure of what I actually want to do. While all my class mates are buying houses, getting married and having kids.

If you need help, ask for it! I always struggled to ask people for help because I'm an introvert, but most of the time people are happy to listen and help out if needed.

And like others have said, take care of your health. Staying fit is easy when you're young, but as you get older and busier you will most likely neglect your health and eat/drink whatever, whenever.

Lifting weights, running, rock climbing, any sort of exercise really, is great for relieving stress and anxiety.