Let's start with describing what mountain biking is, because knowledge is the most important first step in making decisions. Mountain biking is simply riding a bicycle on rough possibly steep terrain. There are two main types of mountain biking:
Cross Country (XC) Cross country mountain biking is the most common type of mountain biking. You should expect some ups and downs. The trail's steepness depends on the level of difficulty you select. XC trails can also be quite flat, and their surfaces range from rocks to slickrock (rolling sandstone) to dirt. This may be the best way for a beginner mountain biker to learn the sport.
Downhill - It could be suggested that you get some time on a mountain bike under your belt before attempting this. Downhill mountain biking is riding your bike down a slope. Sometimes ski slopes are used, and they can get very steep. More dangerous than XC mountain biking because it is faster.
How do I know if mountain biking is right for me?
That said, it is crucial that you know how to ride a bike, and feel comfortable on one. A certain level of fitness is a good idea as well when you approach mountain biking. No need to be able to run a marathon, but be able to walk in the park without getting winded. But it doesn't matter too much what your fitness level is because you can ride at whatever speed or intensity you choose to. You might want to check with your doctor...
Need help finding a local bike shop? Try google. Most of the world does!