This was taken from the IRS site I hope it helps:
You are a sole proprietor if you are the sole owner of a business that is not a corporation. Report your income and expenses from your sole proprietorship on Form 1040, Schedule C (PDF), Profit or Loss from Business (Sole Proprietorship), or on Form 1040, Schedule C-EZ (PDF), Net Profit from Business.

If the total of your net earnings from self-employment from all businesses is $400 or more, you must pay into the Social Security and Medicare systems by filing Form 1040, Schedule SE (PDF), Self-Employment Tax. Self-Employment tax consists of the Old-Age, Survivors, and Disability Insurance (social security) and the Hospital Insurance (Medicare) taxes. For more information refer to chapter 1 of Publication 334, Tax Guide for Small Business.

The Federal tax system is based on a pay-as-you-go plan. Tax is generally withheld from employees wages or salary before they get it. However, tax is generally not withheld from self-employment income. Thus, you may be required to make estimated tax payments. Publication 505, Tax Withholding and Estimated Tax, provides information on making estimated tax payments.