Finally a topic I'm well versed in. I would agree with Archie above in most respects. If you want to mountain bike its best to get one from a bike shop and not a big box store. Also the type of terrain you will be riding will determine the type of bike you should get, but I would recommend something with at least a front suspension, mechanical disc brakes and it should cost somewhere around $500 that is if you want something that will last you years.
Reason for front suspension is that it makes the ride much more comfortable.
Reason for disc brakes are two fold:
1. If you ride when its below freezing out you will need disc brakes if you hope to stop. When water hits the rim of your tire it will freeze and your rim brakes will be completely useless. and yes I am talking from experience here it wasn't fun in fact it was very scary as it was a night ride in 15 degree weather and I had no brakes after breaking through the ice on a puddle.
2. Disc brakes are just that much more powerful, if you need to stop in a hurray you want disc brakes. On a steep decent trust me when I say you want disc over rim again talking from a lot of painful experience there.
As Archie said make yourself a small repair kit at minimum should have multi-tool, small pump, spare bike tube, bike tube patches, tire levers, and two powerlinks (for chain). All of this can be put in a small bag that attaches under your seat so you won't forget it. Always and I mean
ALWAYS wear a helmet if you are mountain biking.
Also if you plan on riding make sure you get some saddle time in before hand, your butt and legs will thank me. Feel free to pm me with any particular biking questions you may have.
My mountain bike is a Yeti 575, knee/shin pads are helpful when riding on the snow and ice being its very slippery
.