Buy a
tire gauge at any
auto parts store, or
department store or wherever.
Make sure the tires are cold--i.e., you haven't driven the car in quite a while, certainly not for long distances.
Take the valve cover off the valve of a tire.
Press the tire gauge onto the valve quickly and firmly so that little or no air escapes from the tire and the tire pressure pushes the gauge out.
Remove the gauge from the tire, and look at how far the indicator is pushed out. Match up the edge of the gauge with the closest line. The number buy that line is the tire pressure in PSI.
To find the recommended tire pressure for your vehicle, look inside the door frame of the driver's side door. There should be a sticker with both the vehicle carrying capacity and the recommended tire pressures for both the main tires and the spare tire, which often is a compact spare and needs a higher tire pressure. Some cars need different pressures in the front and back.
If you're missing the sticker, hopefully your owner's manual has the information. Please note that the recommended pressure doesn't have much to do with what kind of tire is on the car, it's mostly a function of the car's weight and balance--although the recommendations are usually for tires that are the same size as the factory-installed ones. If you change your tire size for some reason, you should know what you're doing.