Are you ready for a serious investment? You're going to need to get into it the same way figure skaters practice all the time, and tennis players practice all the time. The fact that you're already 20 without any real experience will probably count against you somewhat. You can probably make up for it, though.
Start by doing SCCA autocross events and work your way up to SCCA races. The indoor kart track will also be a plus. In the meantime, go to an established racing school (i.e., Skip Barber, Bondurant, etc.) and start taking classes. Start looking for a car to buy that you can race-prep (I've heard the spec-Miata series is relatively cheap and very competitive).
Then, very simply, win. The more you win (or at least get a podium finish), the more you'll get noticed. Then, start polishing your public speaking and presentation skills, as you're going to need them to woo sponsors for dollars. You might want to re-consider any other college, grad school or career plans, since racing is basically a full-time job these days.
Also, look at the webpages of your favorite drivers (*cough* Danica Patrick *cough*) and see all the different series they've had to drive in and succeed at to move on to the next level.
Good luck, keep us posted on your progress. :wink: