To supplement the comment I left regarding TurboTax (http://news.ycombinator.com/item?id=1783200), I thought I'd lay out exactly how and when I interact with the IRS. About 50% of my income comes from freelancing; the other 50% from a full-time job.
January 15: Send the IRS a check for 28% of the gross freelance income you made between September 1 and December 31 of the previous year along with a 1040-ES (http://www.irs.gov/pub/irs-pdf/f1040es.pdf) Also send an estimated payment to your state of about 5-7%, based on your state's income tax bracket for your income. (My state, Minnesota, has easy online payments, so there's no paperwork required.)
Between March 1 and April 15: Do your taxes. Use TurboTax; it will simplify your life. Take appropriate deductions for everything that TurboTax asks, but make sure to have documentation.
April 15: Repeat what you did on January 15, but for income received between January 1 and March 31.
June 15: Repeat January's step again, but for income from between April 1 and May 31.
September 15: Repeat January again, but for income from between June 1 and August 31.
Note that you only really need to make estimated payments if you expect to owe more than $1,000 on tax day if you were to not make estimated payments. If you owe more than $1,000 when tax day comes, you may be charged a penalty if you didn't make estimated payments. (I'm not an accountant, so your mileage may vary.)
Unless it's changed since I last did this sort of thing:
Every quarter you need to file and pay estimated taxes.
Every year you need to file your individual tax form, with your freelancing income minus allowable expenses (be careful here!) in a Schedule C and you'll also need to pay your self-employment tax in a Schedule SE (this is both parts (employer and employee, as the fiction goes) of your FICA (Social Security and Medicare) taxes).
i recommend an accountant. you may need to pay state income tax as well.
The KEY question here is, what are you?
Fed/State tax processes are dependent on what type of business you are e.g. sole-prop, LLC, corp, etc.
Since you don't know how to handle contractor income, I'm guessing you didn't file any business paperwork, meaning you are automatically classified as a "sole proprietor" (key term). Anyone in the U.S. who legally does business without filing any paperwork is considered by the IRS as a sole proprietor.
This means that you can use something called "flow-through" taxation. All profit (or loss) from your business flows through to your personal tax filing. There is no need for a separate filing for your contractor work, just an additional form.
So with that assumption, @hga is right. File a Schedule C EZ (you probably don't need the more complicated, full schedule C) form.
You'll end up paying a special little "Self Employment" tax that is an addition 7.5% on top of FICA taxes, since employers usually cover this.
As far as sales taxes, did you actually sell anything other than your time?
You will, of course, need to pay state income tax, but that is absolutely no different than personal tax. This is because it flows through to determine your AGI, which is then used to determine your state tax liability.
Taxes are actually a fantastic non-technical hacking area. I'm fascinated by it. I also have dated several tax accountants, which I'll admit comes in handy every year as an entrepreneur.