How to put "flats" in sheet music

I am new to working with any sheet music and GB. I am trying to take shhet music that has already been written, and put it into GB so I can play it. What is stopping me is tha the sheet music is annotated with a number of "flats" prior to the time notation. How do I get the "flats" (which I believe is called key notation) into the sheet music editor of GB?