It's called an income tax
Because it's your income that the taxman wants a piece of, start thinking about the employment and income data you'll need to file.
By the end of January, every employee should get Form W-2 from his or her boss showing how much was earned, how much was taxable and just what taxes were withheld. If you have more than one job, you should get a Form W-2 from each employer.
If you're an independent contractor, the company you worked for should send you Form 1099-MISC showing your gross earnings.
When you're self-employed, you have a bit more work to get organized. Track down all receipts and documentation for business-related expenses, from the mileage records you kept when using your car for business to the office equipment and supplies you bought to the utility bills you paid to keep the home office lights on.