Depends how you set up your business. Will you be self employed, sole proprietor, independent contractor OR setting up as a LLC or partnership, etc?
If you don’t set up an official business entity you will file as self employment on Schedule C in your personal tax return and pay self employment tax in addition to regular income tax on it. The SE tax is to pay Social Security and Medicare tax that wasn't taken out like on a W2.
Some general info on self employment on Schedule C.......
You will need to keep good records. You may get a 1099NEC at the end of the year if someone pays you more than $600 but you need to report all your income no matter how small. You might want to use Quicken or QuickBooks to keep track of your income and expenses.
When you are self employed you are in business for yourself and the person or company that pays you is your customer or client.
Turbo Tax Self Employed Tax Hub
https://turbotax.intuit.com/personal-taxes/self-employment-taxes
Here is some IRS reading material……
IRS information on Self Employment
http://www.irs.gov/Businesses/Small-Businesses-&-Self-Employed/Self-Employed-Individuals-Tax-Center
Pulication 334, Tax Guide for Small Business
http://www.irs.gov/pub/irs-pdf/p334.pdf
