I would give them two pieces of advice. First, now's the time to try something new. I don't mean reinvent yourself to someone who's not you, be yourself but try and broaden your horizons a little bit. If you've said, "Oh, I've always wanted to try this," just go for it, because then even if it doesn't work out ... I know a ton of kids who played freshman thirds soccer with me, who stopped playing soccer after that year. But you get to meet new people, you get to try something new.
And then my second piece of advice would be to start good habits early. If you have a class that you’re struggling in, see that teacher on a regular basis. Make sure that if you play a sport and you're doing the play, or you do one or the other, that you get home, you get your homework done at a reasonable hour and get good sleep. There were some times in my sophomore and junior year, that I just had a lot going on, and sometimes if you get in a good routine it's helpful. Even for me, I always do my homework in the same order every night. I'm a person of habit.