This is the tax form you will file for your corporation.  If you don't  know it, don't worry.

If you entered "1120S" after the corporation

must file Form 2553 no later than the 15th day of the 3rd month of  the tax year the election is to take effect.

Until Form (disregarded entities owned by a corporation enter the 2553 has been received and approved,

you will be  considered a Form 1120 filer.