Prepare and be confident in all your equipment, and then get out there and gain experience. It's only natural to be nervous or scared the first few times you do something - that's your body's way of warning you that you're unprepared. But once you've successfully done it a few times, on varied surfaces, you'll know how your bike will react, and how to navigate the terrain. Like any skill, it takes practice.

You could try purposefully falling over a few times in a soft, grassy field - because you probably will fall over a time or two on the trail. Practice "stoppies", braking hard on the front so that you lift the rear wheel and balance solely on the front. That's going to happen from time to time on slow, steep descents, and it's best to have an idea how far you can control it before going over the handlebars. And if you've practiced this in the soft, grassy field, you will get good enough to clip out quickly and literally jump over the handlebars!

The great thing about going from being a road cyclist to a mtb-er is you shouldn't be short of power and endurance. Technical skills may take a little time.