If you are earning money in the United States, then you are most likely liable for taxes. Generally you have two choices as to how you report your income to the IRS and how you file your taxes. You can either be "self-employed" or work for a company. I know programmers who file as self-employed, and I also know programmers who "own" their own corporation or Limited Liability Company (LLC). Either way, there are specific rules that must be followed according to IRS guidelines. You don't want to mess wit the IRS, so I suggest that you hire a good accountant to help you with your taxes.
From "Feeding the Machines" by Brad Schiller, PLSN, Jan. 2010
