Find a skate or allen key that fits your kingpin, which holds the trucks on. Since trucks come in a variety of shapes and sizes, you may need to buy a tool designed by the manufacturer to adjust the tightness.
Tighten the trucks by turning the kingpin to the right. This allows for stiffer trucks. Only turn the kingpin two or three twists per adjustment.
Loosen the kingpin by turning it to the left. This loosens the trucks, giving you more leeway to lean right and left on the board.
Test the trucks until you have the desired amount of give in the board. Place your board on a flat surface and place your foot on the center of the deck. Then move the board left and right. If it feels too loose or too tight, turn the kingpin a couple more times.
Repeat adjusting and testing until you get the feel you want. As you skate on your board, you will need to readjust the trucks from time to time to keep them at the proper level. Some skaters carry a skate key with them just in case their board needs emergency tightening.
Keep the nut flush with the kingpin, so it will not work its way loose as you skate. Special lock nuts are available to prevent kingpins from loosening too much.
Adjust each truck on your skateboard to about the same tightness, unless you have a reason to do otherwise. In slalom, for instance, you need a loose front truck and a stable back truck, so you can keep control when picking up speed.