There are certain fundamental
things you should know while trail riding with a mountain bike
Wear a helmet. Many riders, depending on the difficulty of the ride, will fall.
If you plan on doing cross country riding, I suggest a hard tail bike with front suspension. If you plan on doing major downhill riding, I suggest a full suspension bike.
If you are a beginner, start easy. You will only fall if you ride something that you can't handle.
Make sure that your bike is maintained. Ensure that your brakes, shifters, and gears, are functional.
If the trail is mostly downhill, lower your seat. When riding steep terrain, you will want to scoot your bum back. Do not lean forward, you will only fall.
When riding loose dirt, place your weight to the rear by sitting on your seat.
Carry food, and water. Also make sure that you have tools to fix your bike.
When riding, look forward, and find a line of sight, and follow it.
Pump your brakes. Front brakes should be used in conjunction with the rear brakes, in a light pumping manner, or you may find yourself over the handlebars. If you are going downhill, it is all right to use your rear brakes and slide.
If you ride through a puddle, pump your brakes a few times.
Finally, if you do fall, get back up.