Turbotax said i could file for the rent certification online through turbotax in the beginning of March. So how do I do this?

If you have not yet filed your state return, then you may be able to file for the renter's credit when you complete your state taxes. It depends on the state. Some of them will accept it with an e-filed state return, some require forms to be mailed, and some state's do not have renter's credit at all.

First, you will complete your federal return, then you open your state return and it pulls in information from the federal. If your state has a renter's credit, it will ask questions to see if you qualify. 

This TurboTax Help article lists each state and how they handle renter's credits.