Flip the board upside down, and locate the kingpin on the trucks. The kingpin is the large bolt at the truck center.
Remove the kingpin with a skate key or hexagonal wrench and take off the truck hanger. The hanger is the top piece which holds the axle and wheels.
Pull off the bushing and inspect it for cracks. If the bushing is noticeably damaged, dirty or warped, then it needs replacement.
Replace the hanger and tighten the kingpin back securely. Wiggle the trucks slightly to see if they are secure. If the trucks are loose, tighten the four bolts attaching them to the board, then check again. The trucks should be tight and secure at this point.