Mountain Biking is really fun. As opposed to "10-speed bikes, you can go off-road and down trails and wherever you want to go. There are many trails all over that you can ride on, all for free. There are some things you will need to get started.

The Bike
With so many brands of bikes to choose out there, how do you know which ones to choose? By just looking at them, you can tell if they are good or not. But now adays there are a lot of good companies out there producing bikes so chances are you will get a decent bike. Some things you should look for are dual suspension, which helps for a smooth ride, 21 speeds, and easy shifting ability.

Helmets
Getting a helmet is a good idea if you are doing some serious mountain biking. Take a look at these guys.You can bet they are glad they are wearing their helmets in case they ever fell off. But I doubt you will be doing any serious biking like that. Nevertheless, it's always a good idea to be prepared.

Pads
Again, here is another safety precaution. Pads. If you fall off, you want to be protected. Good pads will prevent bones from being broken and such.

Hydration
The convenience of having the Camelbak cannot be stated enough. The ease of use just makes you glad you have it, especially while mountain biking.