Excel formula question?

This should be an easy one for people familiar with excel. I'm trying to create a billing statement and I'm adapting a template created by someone else. The person who created it has the formulas I want to use in only two rows. I want to apply this formula to probably the next 20 rows in my workbook. How do I make the formula paste into the following rows but change the appropriate cells as it goes along? Haha, sorry for my limited excel vocabulary. For example: In row 18 my formula begins with =IF(E18="Sarah",$C$12 . . . . .etc. I want the E18 to change to E19 for row 19, E20 for row 20, etc. Is there a way to automatically do this or do I have to recreate the formula each time? Any help is MUCH appreciated. If you can explain your answer in simple language, that will be even more appreciated lol.