With braking, I always use my front brake over my back, no matter what kind of trail section I've just entered. Weight placement is also very important in this, where your body is in relation to your bike. If you've just gone into a downhill section and you're braking, push your weight rearwards and you'll be able to pull the front brake that little bit harder, while also preventing yourself from tipping over the bars.
Locking up your back brake is just a matter of learning how hard you can pull on the brakes at certain speeds and on what surface, like pulling harder on road surface than loose gravel, rocks etc.
With jumps and drops and other things like that, start small. Don't rush into huge jumps and just expect it to come to you. Find drops that are maybe only a foot tall to start off with, somewhere like a park, where you won't go tumbling onto rocks and roots or other things like that. Just practise getting your bike to stay flat as the wheels go off of the edge. I used to practise off gutters, only tiny ledges, but just try to pull up and lean back just enough as the wheel goes off the edge that both wheels hit the ground simultaneously.
But still, as it's already been said, confidence is key, and following someone is helpful. I did this the other day riding into a double I'd only ever done once before that didn't exactly end smoothly. Following someone else into the jump/drop/rockgarden/whatever, tells you how fast you need to be going, how to position yourself and everything you need to know before you try it yourself. Once you've done them yourself, you won't be able to get enough of them and you'll be wondering what was so hard.
Hope that helped a bit, happy riding
