I'm not old enough to have that much of a younger self, but here goes:

-Eat less. For god's sake.

-Seriously, put less sugar in your diet (I still need to do this in the present...)

-For crying out loud, practice. Just, practice.

-Don't spend so much time doubting yourself. 90% now is better than 100% never, and if you don't just start doing it, never is when it'll happen.

-Once you get a handle on Linux, ArchLinux is the way to go.

-Learn Scheme. It's fantastic.

-Keep track of those old disks. You'll regret losing them one day.