One of the most difficult decisions I have ever had to make was the purchase of a mountain bike. There are so many models to choose from, and my budget was very limited (college will do that to you).
In most cases, you get what you pay for. Before purchasing a mountain bike, you should see how much you are willing to spend. A mountain bike can cost anywhere from 100$(Wal-mart) and up.
Secondly, what kind of riding will you do? In most cases, you will need to select a bike appropriate for your riding needs, for example, if you intend on downhill or trail riding, you may want to consider a full suspension model. If you plan on cross country riding, then a hard tail with front suspension would be a good investment.
I have also learned that older bikes, like certain models of the Ibis bike, do not have suspension, however, the front fork is designed in such a way that it acts like suspension.
A lot of people consider weight a factor. Weight should be the last thing to consider. Heavier bikes tend to be stronger. Aluminum and Carbon will suffice and tend to be lighter. Chrome moly is a mix between Aluminum and steel and provide a stronger "in between" metal. Rocky Mountain bikes are famous for using Chrom moly. If you tend on trashing the bike, get a strong frame.
As mentioned before, you get what you pay for. Avoid cheap components. They will break. On most mountain bikes, you should look at the suspension, tires, brakes, pedals, and shifters/derailleurs. If your pedals require special shoes, intend on taking a week to learn how to use clipless pedals. Consult with your bike dealer or use the Internet to find out information about each component.
Try before you buy. It is very important to see if you're comfortable riding the bike to be purchased. Try out a few similar models if you can. I ended up spending $200 more for a different bike because it felt comfortable. It was worth it.