Spin classes are an excellent way to condition for cycling although won't improve your skills for mountain biking (it doesn't hurt however, it's all good). MTB riding is all about technique and strength and the best way to learn all that is to get out and ride. I would start out on flat trails and get comfortable on your bike and then move up to other, more techical trails. Look for hilly trails to climb. The sooner you start riding steep hills the better. Look to see if there is a MTB Club in your area. Some of them might sponser "Newbie" rides and also look for ride clinics. Alot of areas sponser ride clinics to teach skills. Also some of the LBS made have clinics or classes. Do you have friends that mountain bike? Would they be willing to ride with you and coach you? I think most of us learned skills from our friends.