GO EASY ON THE BRAKES
You will be tempted at some point as a beginner at off-road riding to grab both brakes and pull them tightly. Resist this temptation; MTB brakes are powerful enough that you need just one, maybe two fingers to modulate your speed.
Adjust your speed before the tricky stuff, like corners and then maintain your speed through them. If you do find yourself going into a turn too fast, stay off the front brake. Stopping your front tire will send your front tire into a slide, which is likely to throw you over. Hit the rear brake instead, you might skid but you are more likely to stay upright.