Buying a new mountain bike can be confusing. There are several different brands of mountain bikes, and several different types of each brand. Always buy a bike for what you are going to use it for. We all know downhill and freeride bikes look awesome, but if you’re going to be riding cross country, buy a cross country bike. Now there are two types of cross country bikes hard tail and full suspension. Neither one is better than the other. The deciding factor is how they feel to you personally. Full suspension is better for long rides because your butt wont hurt as bad, and hard tail gives you a feeling a being better connected to the ground and they feel a little more responsive. But once again, it is all your personal preference.
One thing that many people do not know, is that most bikers in the same price range usually have the same components. The main brands are shimano and sram, both of which are great companies. Since all of the components are about the same, the most important thing is the frame. The frame needs to be strong, somewhat light, but most of all it needs to feel good to you. The most important thing to do is TEST RIDE! Try out as many different bikes as you can, then once you find one you really like, try a few more. If you do this I promise you will not be disappointed with your purchase. Many bike shops will let you rent the bikes they have and go try them out on the trails. I would definitely take advantage of this.