Trail running can be intimidating for new runners and even experienced runners who have never run trails. My advice is just get out there and try. I can guarantee you will love it. It is different and harder in some respects. Go out with minimal expectation of your performance when you first start, just like you did when you began running roads.
Trail running takes practice. The more you jump rocks and descend crazy slopes the better you are going to get. That being said, there are some tricks you can start with. When you are running uphill don’t tip your hips forward or back too much. Try to keep your torso straight up and down, keeping your chest up and open. Shorten your stride and pump your arms. Stay up on your toes as much as you can. Sometimes the mountain is so steep it is faster to walk up than try to run. If that is the case, don’t waste all the energy trying to run it, you look silly anyway as hikers pass you trying to power up the hill.
Going down can be more difficult than going up. You want to lean into the forward momentum. Your feet will slip out less if you keep your weight on your toes on steep descents. Landing on your heel is like putting the brakes on and your feet will slide right out from under you. Keep your stride short and fast. If there are a lot of rocks, keep them really short and fast. If the rock looks loose, it probably is.
Put your arms or elbows out for balance, and be ready to change your footing quickly. I try to keep my hands empty on crazy descents. That way if I fall I don’t have to search for whatever I chucked or replace what I break.
When you’re trail running watch the ground in front of you about two feet and then farther down about 10-15 feet, so you know what is coming. Plan your foot strikes as much as possible and be ready to correct it if you touch down and find it precarious.
Make sure and give yourself enough time to finish your run. Trail running takes longer because of the climbs and more technical terrain, especially when you are first starting out.
I’ve watched some of the top ultrarunners (Anton Krupicka and Killan Jornet) come down steep descents and it is terrifying and mind blowing all at the same time. It takes practice, like everything else in running. Don’t go out there thinking that your speed and skill on the road will translate onto the trail because it doesn’t.