How to Jump a Mountain Bike
To jump your mountain bike, start by cycling toward a ramp at moderate speed, riding out of the saddle. When your front wheel hits the edge of the ramp, stop pedaling, lean forward, and use your hands to push down on the bike's front suspension through the handlebars. Get into a crouch, but don't lean past the handlebars, since that can make your bike pitch forward. As your back wheel reaches the edge of the ramp, explode into a jump by moving into a standing position, pulling the front of your bike up as you shift your weight from your hands to your feet.
Make sure your bike is in good condition. This is an important part of mountain biking in general but is essential for you to safely jump. Your landing will place much more force than usual on its frame. Additionally, malfunctions become much more dangerous if they happen while you are airborne. Do a safety check before every ride. If you're not sure your bike is in good condition, visit a bike shop to get it checked out. Be ready to jump. You should practice biking for a while before trying to jump. If you are inexperienced, you are likely to fail the jump and get yourself injured. A successful jump also requires a certain amount of general physical fitness. Keep your body in shape before jumping. Consider lowering your saddle below its normal level before trying jumps. Otherwise, you may hit your crotch on the saddle during your jump, which can be very painful. Visualize your jump. Before you attempt any kind of jump, look at your ramp. Plan your route to it and picture landing on the other side. A general rule of thumb is that if you can't see yourself landing safely, the jump should not be attempted. If you see a lot of rocks and debris along your route and on the other side, find a different place to jump. Practice jumping using a man-made ramp that is gently sloped and low to the ground. This will allow you to perfect your jumps in a safe, simple environment. Once you have mastered safer jumps, you may choose to use more challenging ramps. Popular ramps for mountain biking are man-made dirt jumps and natural outcroppings. Bike towards your ramp. Keep your bike at a moderate speed, riding out of the saddle. You don't want to go so fast as to make a mistake, especially when you're first learning. On the other hand, if you go too slowly you will not be able to clear the jump. Compress your bike once the front wheel reaches the lip of the ramp. Stop pedaling. Push down on the front suspension through your hands on the handlebars. Crouch slightly forward. However, don't lean past the handlebars, which may cause your bike to pitch forward. Do not return to your seat until after you've finished your landing. Explode as the back wheel reaches the lip of the ramp. "Exploding" is the term used by cyclists to describe when they stand up straight after compressing, pulling the front of the bike up as they jump. You will be shifting your weight from your hands backward to your feet and subsequently the second wheel. Shift your posture from one that is bunched up to one that is nearly standing. Keep your hands on the handlebars and your feet on the pedals at all times. When you do this, you're basically jumping up from a crouched position and pulling the bike with you. Enjoy your flight. If you have properly compressed and exploded, you should now be airborne. Take a moment to experience one of the most entertaining parts of biking. Keep your body loose and avoid stiffening up. If your bike did not leave the ground or you got off balance and fell, review the steps and try again. Return to your ready position. The ready position is the default stance you take while cycling. Keep both your elbows and knees bent and out while riding out of the saddle. Do this before you land. Land on both wheels simultaneously. Make sure your bike is straight with the wheels parallel to the ground while you are still in the air. Shift your weight to level out your bike. Landing with both wheels at once will help absorb the shock better and keep you from washing out. Push the bike towards the ground. Pushing the bike into the landing will create more traction, allowing the wheels to rebalance and grip the ground more easily. Use your arms and legs as your bike's primary suspension. Keep them loose and ready to soak up the impact. Roll out of your landing, rather than braking. Jumping your mountain bike produces a lot of momentum, so it's important that you don't brake right after you land. Otherwise, you may flip over your handlebars. To safely come out of your jump, keep rolling forward to a slow stop. After the jump, your momentum can carry you pretty far. 
Make sure your bike is in good condition. Be ready to jump. Visualize your jump. Bike towards your ramp. Compress your bike once the front wheel reaches the lip of the ramp. Explode as the back wheel reaches the lip of the ramp. Enjoy your flight. Return to your ready position. Land on both wheels simultaneously. Push the bike towards the ground. Roll out of your landing, rather than braking. 