Once you've inflated all tires to the recommended pressure (check the sticker on the driver's side doorjamb or your owner's manual), drive the vehicle for a short distance (a few miles). The system should then automatically detect the correct pressure and turn the warning light off.
If the light remains on after inflating the tires and driving, there might be a problem with one or more of the tire pressure sensors, a faulty sensor wire, or a problem with the system itself. You should have a mechanic check the system to diagnose the issue.