Conditional Formatting

Here is my chart:

Amount Paid	Paid (Yes/No)
$100	YES (color yellow)
	NO (color red)
$400	YES (color yellow)

How do you make an if then statement, so that when the cell is blank in column "Amount Paid", it has the word "NO" and the cell is shaded red.

Then when I put any amount in the column "Amount Paid", it changes to YES and the cell is shaded yellow.

If someone could please help me with this formula, I would be much appreciative.

Thanks!