IF elif statement. Help I'm at my witts end?

How would I do an if, elif statement if

e.g. if row 'A' is empty then row 'C' = "N/A" else if row 'A' is not empty then row 'C' = (row A - row B and how would I get this to go down every row of the spreadsheet?

I've tried some examples online but I'm completely lost as these don't seem to give me what I'm looking for.

Thanks in advance