Yes you have to fill out schedule C to report your self employment and 1099NEC income. Even if you don't get a 1099NEC you have to report all your self employment income. Then you enter all your expenses. You will pay Self Employment tax on a Net Profit of $400 or more. That doesn't have anything to do with the SS benefit checks you are getting now.
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. The SE tax is in addition to your regular income tax on the net profit.
Turbo Tax Beginners Tax Guide for the Self Employed
https://turbotax.intuit.com/tax-tips/self-employment-taxes/beginners-tax-guide-for-the-self-employed...
Here is some IRS reading material……
IRS information on Self Employment
https://www.irs.gov/businesses/small-businesses-self-employed/self-employed-individuals-tax-center
Publication 535 Business Expenses
https://www.irs.gov/pub/irs-pdf/p535.pdf
Publication 334, Tax Guide for Small Business
https://www.irs.gov/pub/irs-pdf/p334.pdf
1040 Schedule C
https://www.irs.gov/pub/irs-pdf/f1040sc.pdf
1040 Schedule C Instructions
https://www.irs.gov/pub/irs-pdf/i1040sc.pdf
