Start by looking at the sticker on the door jam. If it's gone, look at the owner's manual.
Just as a rule of thumb, I wouldn't go less than 30#. If the maximum inflation pressure (on the d\side of the tire) is 44#, use 40#, Try to keep around 2 to 4# difference between the maximum and the pressure you inflate them to. Remember, when the temperatures go up, you tire pressures will too.