Checking tire pressures on the average vehicle is simple. All you need is a pressure gauge.

Most tires have a maximum rated pressure of about 40-50 PSI (Pounds per Square Inch), but most vehicles only require about 35 or so - check the sticker in the glove compartment or on the doorjamb of the driver's door to find yours.

Now that we know the proper pressure, we can proceed to checking the tire. Somewhere on the rim of the tire there will be a valve stem, usually covered with a little screw-on cap. Unscrew this cap, and set it aside. I'll assume for the moment that you have the 'stick' type of pressure gauge. Press the business end of the gauge against the vale stem such that you hear a rush of air. Continue pressing harder until the sound stops. If the sound doesn't stop, either you're using the wrong side of the gauge, or your gauge is broken. After the whooshing sound stops, check the reading on the gauge. This is your tire pressure.