Starting out is pretty easy. First thing you want to do is register for a business license in your state/county. At the same, register your trade name. I would also suggest that you register as an Limited Liability Company (LLC) as it will protect your personal assets. Next step would be to register for a Federal Tax ID, which is required for other steps in starting a business, such as opening a bank account etc. Check out:
http://www.bizfilings.com
This site is dedicated to helping small businesses file legal paperwork at low cost.
If starting a venture does not interfere with school, I say go for it. At this point, your education should be your first priority. But starting your own business at 19 really gives you a head start.
Good luck!