Where does White have pieces vulnerable to a pawn fork? He has pieces on c3 and e3, and Black has a pawn at d5, so the makings of a fork are in view. There are two obstacles to its success: the White knight on d4 is in the pawn’s way and will need to be cleared somehow; and if a bishop is going to be one of the targets of the fork, the pawn will need protection so it doesn’t get captured. Once you understand that those are the problems you need to worry about, the solution is clear enough: play c7-c5. The threat drives the knight away (every piece flees a pawn), and also creates protection for the d5 pawn when it then moves to d4, forking knight and bishop.

It's worth having a good look at the starting position here. It's important to see the potential for a pawn fork in a situation like this despite all the other distracting pieces in the vicinity—particularly the knight on d4.