i filed my w9 for when i helped work at a resturant i previously worked at but now i also need to file my 1099-nec form for the same year, how do i do that

If you filed a tax return already and did not include ALL of your income, you will need to Amend Your Return.  When you file a return, you need to include ALL of your income from ALL sources on one return.  You should not submit your return until you have all the income entered on the return. 

It is best to wait until the IRS has fully processed your return before you amend as it is possible that they will catch the omission and send you a letter with the correction on it.  If they do, then you will just follow their instructions in the letter if you do not agree.  If you do agree, you will do nothing, unless you owe taxes, then you will pay the amount due.  If they do NOT catch it, then you will amend your return.