You need to file Form 2553 with the IRS before you can file your taxes as an S-Corp.
If you want to elect S corporation status, you should file Form 2553 to change your business to an S corporation either during your corporation’s first tax year for which you want to make the election or within the first 2.5 months following that first tax year
As an example: If you are on a calendar year, you would have to file Form 2553 by March 15 of the year when you want the election to take place.
Click here for "Instructions for Form 2553 (12/2020)"
Click here for "What is Form 1120-S? A Guide to the US Income Tax Return for S Corporations"
Click here for "How to File Federal Income Taxes for Small Businesses"