If you find that you really enjoy this sport and enroll it into your lifestyle as one of your main activities, you can always upgrade your bike in the future. Many enthusiasts have multiple bikes for different trails. Here in Northern California, we have many types of trails to choose from. Some are relatively easy with no obstacles all the way to steep descents, rock gardens, rooty single track and ruts.
Based on the trail, you can select which type of bike you need that best suits the terrain. For example, you would not want to take a fully rigid bike to a place that features highly technical riding. Idealy, you would want to take an 'all mountain' bike with 5 inches of rear travel. A bike like a Santa Cruz Nomad or an Ibis Mojo would be ideal. All downhill bikes would be great to take as well, but they are heavy which makes it harder to climb.
You may eventually want to get into Cross Country racing (XC) or maybe even ride 'Downhill'. There are bikes for every type of rider, skill level and price range.
There are several factors when shopping for a Mountain bike as there are a various types, each one has it's own characteristics and benefits for the rider and type of trails that you will be riding on.
Hardtail
A 'hardtail' bike is a frame that has no rear suspension and is much more efficient when you are climbing uphill, but is more unforgiving on the decent, especially if the trail has a lot of obstacles like tree roots, rocks and ruts. You will see that most Hardtail Mountain bikes will be built with a front suspension fork which is a standard today.If you select this type of Mountain bike and have technical trails, you will eventually learn to choose the smoothest line down the trail to avoid the rear of the bike from bouncing around. Another benefit is that they can generally be built lighter than a full suspension bike which bodes well on long climbs.
Full Suspension
A full suspension bike has a front shock and a rear shock to provide a smoother ride over rough terrain. They have an advantage over a hardtail on technical descents as the rear shock can absorb obstacles and provide a much smoother ride. In general, the Full Suspension Mountain bike is heaver than a hard tail build which makes climbing a bit harder. Some cross country racers use a 4" travel XC bike to race if the course has a lot of technical descents. 
29'er
The 29'er is a larger wheel sized Mountain bike that is growing in popularity over the last few years. The overall size of the Mountain bike is relatively the same, but the wheels are larger as the rim itself is 29 inches compared to a 26 inch standard rim. The 29" wheels allows the user to roll over obstacles like rocks, roots and ruts easier than a 26" tire because of it's circumference. More manufacturers are starting to produce these bikes and there are more full suspension 29'ers making their way into the market.
Single Speed
This particular bike is a hard-tail and is usually built from the frame up. There is one chain-ring up front which is usually a 32t (teeth) and the rear sprocket is changed out depending upon the trails that you ride and your fitness level. The amount of teeth for the back sprocket is about 1/2 of the front chainring. If you have a 32t in front and divide by 2, you'll have 16. You will need to test different sprockets in the back to find a suitable gear ratio for the trails in your area.
Single Speed Mountain bikes are growing in popularity as it is very basic which is what people like about them. They have less mechanical issues on the trail as they do not have multiple chain rings and derailleurs. They can also be built very light which makes climbing much easier. (No derailleurs, shifters, extra sprockets and chain-rings). A sub 20 pound Single Speed bike is common.
People that are on Singlespeeds are usually experienced riders with a few years of riding under their belts, so it's not the best choice if you are just starting out.
After you become familiar with your trails and are relatively fit, you may want to consider building up a Single Speed Mtb.
What types of trails will you be riding? (In general, this should determine which bike you need)
Relatively flat with small climbs and descents, mostly fire roads.This particular bike is a hard-tail and is usually built from the frame up. There is one chain-ring up front which is usually a 32t (teeth) and the rear sprocket is changed out depending upon the trails that you ride and your fitness level. The amount of teeth for the back sprocket is about 1/2 of the front chainring. If you have a 32t in front and divide by 2, you'll have 16. You will need to test different sprockets in the back to find a suitable gear ratio for the trails in your area.
Single Speed Mountain bikes are growing in popularity as it is very basic which is what people like about them. They have less mechanical issues on the trail as they do not have multiple chain rings and derailleurs. They can also be built very light which makes climbing much easier. (No derailleurs, shifters, extra sprockets and chain-rings). A sub 20 pound Single Speed bike is common.
People that are on Singlespeeds are usually experienced riders with a few years of riding under their belts, so it's not the best choice if you are just starting out.
After you become familiar with your trails and are relatively fit, you may want to consider building up a Single Speed Mtb.
What types of trails will you be riding? (In general, this should determine which bike you need)
Recommendation - Hardtail with 80mm travel front fork. If you are really set on a full suspension bike, look for a 80+mm travel front fork, 4" rear travel cross country (XC) bike. If you will not be riding technical trails with obstacles, you really do not need full suspension. It will smooth out the overall ride.
Rolling hills, single track with some technical parts that include some rocks, roots.
Recommendation - Hardtail with 100mm travel front fork. Or a full suspension, 100mm travel front fork with 4" rear travel cross country (XC) bike.
Mountains with single track and technical riding including large rocks and roots, obstacles over 1 foot in height, drops over 1 foot.
Recommendation - Full Suspension, 120+mm front fork with 5" to 6" rear travel (All mountain)
Mountains with steep descents, accents, large rocks, stunts, jumps with major technical sections - Recommendation - Full Suspension, 140+mm double crown front fork with 7" to 8"+ rear travel (Downhill Mountain bike)
This is a general guideline, go to your local bike shop and ask them about the bikes that they typically sell in your area and for the trails that you ride. Send me an email and I'll be happy to answer any questions that you may have.
