Leo Babauta writes a letter to an 18-year-old searching for direction and includes a lot of really great advice:
Teach yourself stuff. The Internet has anything you want to learn, from writing to 3D animation to programming to carpentry to guitar. Never stop learning.
Find out how to motivate yourself. There will be times when you don’t feel like doing anything. This is a good problem to have, because you’ll have to figure out how to solve it or else go get a boring job where someone motivates you. Solve it. You’ll be much better prepared for the road.
Figure out what you’re passionate about. This isn’t easy, because it takes a lot of trial and error. Try a lot of things. When you get good at something, by the way, you’ll like it much more. You’ll suck at everything at first.
I love so much of this — and I’d add to it: read. Read as much as you possibly can about everything. Things that interest you, things you want to learn about, fiction, non-fiction, poetry, stories, and especially things that challenge you and how you think.