Any advice for someone trying to learn how to mountain bike?

I recently bought a Diamondback Coil EX mountain bike. It is a mid level priced bike, around $500. I didn't want to spend $1000 or more because I haven't ridden a mountain bike in 7-8 years and the bike I had before was a top of the line Wallmart bike...LOL...so this is officially my first real mountian bike. I bought it because there is a group of people at my church that go out riding every week and I want to join them. I was a BMXer from age 4 until I was in my 20's so I feel that mentality is going to be my first big obsticle. I went riding yesterday and I had some difficulty on the hills. I would gear down to the lowest gear on the steep hills and would be doing real good about 20-30 feet and then my wheel would spin. Going down hills I am hard on my brakes. Any advice on how to get better as a new mountain biker?