The income from your 2020 side job will go on your 2020 return next year when you prepare it in 2021. You will pay some self-employment tax as well as ordinary income tax. If it is not a lot of money you may not need to pay any estimated tax---if you expect to owe at least $1000 at tax time then you should pay some estimated tax during the year to avoid an underpayment penalty at tax time.
https://ttlc.intuit.com/questions/2926899-how-does-my-side-job-affect-my-taxes
https://ttlc.intuit.com/community/self-employed/help/what-is-the-self-employment-tax/00/25922
https://ttlc.intuit.com/questions/2902389-why-am-i-paying-self-employment-tax
https://ttlc.intuit.com/questions/1901110-do-i-need-to-make-estimated-tax-payments-to-the-irs
https://ttlc.intuit.com/questions/3398950-what-self-employed-expenses-can-i-deduct
https://ttlc.intuit.com/questions/1901340-where-do-i-enter-schedule-c
Next year when you prepare your 2020 return if you use desktop software, all the forms and schedules are in desktop software so that you can prepare the schedule C you will need. If you use online TurboTax you will need Self-Employed.
If you have already reached the 2020 limit for Social Security withheld with your W-2 job, then when you do your tax return you may only owe SE tax for Medicare.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
