If you "side job" is self-employment, that is treated as a business, and subject to business taxes.
If instead, your "side job" is working for a employer as an employee, you would probably have received a W-2 form from that employer.
If you are self-employed, you may wish to refer to IRS Publication 334, Tax Guide for Small Business at
https://www.irs.gov/pub/irs-pdf/p334.pdf
I hope this helps.
