My grandmother passed away in January 2018. On T1 there is a question on first page "is this return for a deceased person?". Where in Turbo Tax can I enter this info?

I am sorry to hear of your loss.

Right at the start of the interview, there is a question that asks “Who are you preparing this return for?”. If you answer “My spouse/partner and me” or “Someone else” it gives you a check box to indicate that someone has passed away. Unfortunately, once you pass that point in the program, you can go back to change your answers, and must start a new return to input the correct options.

However, in your case, your grandmother passed away in 2018. So for her 2017 tax return, she was still alive, and you would input your answers based on that. When you do her 2018 return, that would be her final return and you would indicate her passing at that point.

Here is some more information regarding preparing returns for the deceased: https://turbotax.community.intuit.ca/questions/1189877