Welcome to the world of mountain biking. I'm sure once you get the hang of it, you'll love it!

All the advice already provided is really good, so I won't repeat it. The really important thing that was noted was to not look at the rock you're trying to avoid. Look where you want to go, not where you don't want to go. Also, make sure you're looking ahead and picking the best/smoothest line at all times.

For sand, DON'T USE YOUR BRAKES!!!! It's very tempting, but you have to learn to not use your brakes in the sand. If you see sand ahead and want to slow down, hit the brakes before going in the sand. Try to float right through it, use your body weight, not the handlebars, to navigate. Also, shift down to spin quicker through the sand helps a lot. You don't want to be in a hard gear, you'll lose traction!

Good luck!