Getting used to the bike can be one of the biggest challenges when it comes to mountain biking. Take some time to get used to the bike before you hit the trails. Make sure your saddle is set to comfortable height, get acquainted with your gears and get used to the brakes.
Mountain bikes are equipped with a different breaking system (disc brakes) to commuter or road bikes. They are usually far more sensitive to your touch. You should refrain from pulling the break in one hard swift movement and instead pulse the breaks lightly. While the front breaks will prevent skidding, if pulled too hard you will find yourself face down on the trail having been catapulted over the handlebars in a rather dramatic fashion.