First you have to decide what you really want to do with it.
Mountains bikes aint just mountain bike any more.
There are at least 6 groups:
Cross country hard tail
Cross country duel suspension
Cross country are light and fast but cant handle the big hits, typicaly they only have about 2-3 inches of travel front and rear and often dont have disc brakes.
Jump/freestle have front sus but generally no rear. The frames are built like tanks and they usually only have a rear derailer.
Great for stunting and jumping but crap to pedal up hill or go fast down.
All Mountain are a beefed up cross country duel suspension.
Usually 5 plus inches front and back with 3 front chain rings so you can actually pedal them up hill, but they are heavier than CC so they can handle the bike hits
Black diamond are some where between all mountain and down hill. Harder than all mountain but softer than down hill.
Down hill do just that and that only, go down hill fast.
Long travel sus [up to 9 inches now!] and big hydralic disc brakes but they weight in excess of 45lbs often and you are dreaming if you want to pedal up hill.
If you dont have any really gnarly trails [big roots, rocks, drops etc] in your area maybe a cross country duel sus would suit.
I have a Mongoose Teocali all mountain with 5 inches front and back and discs. For the tracks I ride any thing less would be totaly masochistic.
Brands are not really relevant IMO [with the exception of anything purchsed from Kmart
Its is the group set or components that your spend your money on ie brakes, cranks, pedals, gears etc].
All bikes have components from either Shimno or SRAM
It is only the frame that the bike manufacturer actually designs and builds
Shimano components go up in quality and price from deore, LX, XT, XTR
Sram goes X5, X7, X9, X0
X0 and XTR are consider the best but are made very light for racing and can be fragile - considered to be bike porn
Spent as much as you can afford and use this web site to check the reviews of you final list of models
I hope this helps