VBA If Then Statement with Loop

Ok I am new to VBA and not too familiar with how looping works.. I've got what seems like a pretty simple application with looping and If..Then statements but need some help on structuring it.

In very a basic example, I have a list numbers in column A and the values PM or AM listed in column B. I want to write a loop that will search every value in column B until the end of the data set, and add 12 to each value in column A each time column B has a value of PM. In a nutshell, it would look like this:

-If column B = PM

-then add 12 to its corresponding cell in column A

-else move down to the next row and do the same thing until you reach an empty cell

Any assistance would be greatly appreciated!