Righto, benefit of the doubt. These are words for beginner not gnarly ex moto type lunatic
1. Find out what kind of bikes in terms of suspension travel your mates are riding and whether they are duallies or hardtails
2 if you are over 6 foot - get a 29er
3 if under , 27.5 or 29er, if positively small ( under 5'6"), do not go near a 29er
4 26ers are fine, but look like been replaced by 27.5 which is the same as 650b.
5 look for a trail bike - 110 through to 135mm of travel, once you get above 150mm they are a bitch to pedal and hence keep up with fitter mates - as a beginner you won't be flying down mountains anyway, so the long suspension travel in all mountain 160mm type bikes will be lost on you.
6 in order of value off the shop floor giant, norco, avanti, merida, then few hundred more to trek and specialized
7 only look at models with air sprung forks.
8 on a new bike I would only consider a bike with a shadow plus deraileur or a type 2 ( these are the clutch type deraileurs, and any bike specifier with any idea would automatically include these as a bare minimum for offroad use)
Second hand - only buy off someone who knows you or knows a mate, and who is trustworthy, and preferably not hard up for cash
Conclusions - as a starter bike that will last, the easiest recomendation is to get a trance and pick wheel size based on height and always last years model given the tight budget . Best warranty around and most
Y good value
Didn't get the hardtail part - basics still apply, just $500 cheaper