Having gone the route of independent, corporation, etc. - if you are just doing some freelance work, you can start virtually immediately earning Schedule C 1099 income as an individual.
However, word of caution, get an accountant quickly, pay your estimated taxes monthly (not quarterly) - you can do it online at EFTPS.com - 1040ES form.
Then, you won't be "tempted" to spend the money you earn and owe a big tax bill. Depending on your regular income and what you earn freelance, plan on taking 25-30% out right away.
Also, setup a simple set of envelopes for receipts - even a single envelope, and use your checkbook software or quickbooks to categorize your financial transactions. It will save you time and money when your accountant prepares your taxes.
Finally, don't do your own taxes. Really. Focus on those things that maximize your income.
Matthew Moran (career blog and podcast below)
Career Advice with Attitude for the IT Pro