first go to a bike shop and find the right size frame and have them show you how to tell if it fits properly
then look for a used bike
name brand, something you have heard of and isnt crap
shimano components, try to find deore, lx, or xt
decent front shocks, name brand, some type of adjustment to them

i paid $800 for a used specialized stumpjumper m2 fs, xt components, judy xc shocks back in 1998
today i could sell that bike for maybe $200 and i have put another $300 or so into it in the last few years

find something 5 years old that was midlle line ($1000) then, but isnt worth shit now, it fits well, it is easily upgraded, it will outperform new stuff thats more expensive, but it isnt as fancy as newer stuff, just alot cheaper

also a good frame is the best place to start, anything else can be changed, but a frame that fits well, is light, and is strong will put you on a good path

i need to get my bike back out, its been too long