Usually, an appropriately sized mountain bike will have at least 2 to 4 inches of room between the top of the frame and your crotch when you stand above the bike. You should be able to reach the handlebars such that you can bend over at a 45 degree angle without feeling cramped. The height of the stem should be adjusted to put the handlebars slightly below the saddle, allowing some of your weight to shift to the front of the bike for a more balanced position. The saddle should be set at a height so that your legs reach almost full extension when you place your heel on a pedal in its lowest position. Bikes with front or front and rear suspension are becoming more popular and more affordable.
Suspension smoothes the bumps in the trail, increasing comfort and decreasing fatigue.
You may want to consider a bike that comes with suspension, or one that can have suspension added at a future time. If you are unsure about the way your bike fits you, talk to the experts at your local bike shop. They should be able to help you properly adjust your bike. When buying a bicycle, it is best to try several different bikes before settling on one. Buy one that fits right and wear it.