You pay self-employment tax when you file your tax return; you may also pay quarterly, depending on your situation. Self-employment tax is 15.3% for Social Security and Medicare. You also pay ordinary income tax. When you are self-employed, you complete a Schedule C to enter your expenses.
To report your self employment income you will fill out schedule C in your personal 1040 tax return and pay SE self employment Tax.
Self Employment tax (Scheduled SE) is automatically generated if a person has $400 or more of net profit from self-employment. You pay 15.3% SE tax on 92.35% of your Net Profit greater than $400. The 15.3% self employed SE Tax is to pay both the employer part and employee part of Social Security and Medicare. So you get social security credit for it when you retire. You do get to take off the 50% ER portion of the SE tax as an adjustment on the 1040. The SE tax is already included in your tax due or reduced your refund.
Here is some IRS reading material……
IRS information on Self Employment
Publication 334, Tax Guide for Small Business
Publication 535 Business Expenses