Buy a tire pressure gauge to check the pressure in your tires. Find an auto parts store, such as NAPA, AutoZone, Checker, Kragen, etc. In general, dial-type gauges are much more accurate than pencil-type gauges.
Use a manual floor pump such as a bicycle pump (schrader-valve compatible) or go to your nearest gas station. Many gas stations provide free air if you purchase some gas. Make sure your tires are cold (i.e., you've driven less than 2 miles on them). Pull your car alongside the air dispenser. It's usually located on the outskirts of the gas station's parking lot. If you don't see it, ask the attendant.
Consult your car's manual or your driver-side doorpost for the recommended tire pressures. Some vehicles will require the same amount of pressure in every tire, but others will have different values for front and rear tires. Check the current pressure in your tires.
Add air if the reading is lower than it should be, or bleed air if the reading is higher than it should be. (To bleed air, press the center pin in the valve stem with a fingernail or tool).
Replace the valve stem cap. Repeat the process for each tire.