The best way to get started is to join a club. A good club will accept you as a newb. The best way to learn mtn biking is to go mtn biking. It's a whole new world and has its beauties. It's not like roading, it's more all out, then nothing, then all out again. The better equipment you can afford the more you'll enjoy it. And no, a walmart bike won't last long on moderate trails. It depends on how serious you get, but a dual suspension bike is nearly required if you're going to tackle technical trails. You can get a decent duallie for around $1,000, less for a used bike.