Mountain bike forks are designed with the weight of the rider in mind. The pressure of the air pockets inside a suspension fork can be changed to accommodate riders of almost any size and weight, so that anyone can use the fork to its full potential. The setting of this pressure is called "sag adjustment" and should be performed once when you purchase a new mountain bike, then once every few rides to be sure the shock stays in adjustment.
Remove the air valve cap of your fork, usually located underneath the left fork leg. Some suspension forks have two, one for each leg. Set the sag gauge that came with your fork beneath the rubber band in place at the lower part of the fork crown, where the smaller and larger tubes of the fork meet. if you don't own, or have misplaced, your sag gauge, you should be able to purchase one from your local bike shop.
Hold both brake levers and sit on the bike in your normal riding position. The fork will sag and the rubber band on the fork leg will move in accordance with the fork travel.
Get off of the bike and investigate the position of the rubber band relative to the marker on the sag gauge next to your weight. If the rubber band sits higher on the fork than the marker on the gauge, connect your suspension fork pump to the valve beneath the fork leg and add 10 psi (pound per square inch) of pressure to the fork. If your fork has two valves, add 5 psi of pressure to each fork leg. If the rubber band sits below the marker, remove 10 psi of pressure from the fork.
Remove the pump and push the rubber band back to the bottom of the fork leg. Hold both brakes and sit on the bike again. Get off and investigate once more the rubber band's position. If it still sits too high or too low according to the gauge, add or remove another 10 psi from the fork and try again. Repeat this process until the fork sag is just right.
Reinstall the valve cover(s) on the fork leg(s) and remove the sag gauge from the fork. Take the bike out for a test ride, and experience your bicycle the way it was meant to be ridden. An in-tune fork is more responsive and comfortable, promoting confidence in the corners and over large obstacles. Re-inspect your fork sag settings every couple rides to keep your bike riding great.