work out what you want. Ride a few freinds bikes if possible, or shop bikes are see what your preferences are, trail bike/xc racebike/ etc and take it form there.look for value for money, ridablilty, upgrade prospects. If going cheaper get a bike with the same or similoiar frame ( maybe same geometry but different heavier materials) frame as a higher end model is always good.If expensive then more options. The frame is the most important item, followed by forks and wheels imho. Also one of the best criteria is fit, ie top tbe length and can be worked out through tech info of bikes and trying it to see if it feels RIGHT.If you cannot do any of this then you proably won’t go far wrong getting a bike ofF the bigger companies and reading mag reviews. Fit however is still important, so be prepared to put some thought into that however rough it maybe.Comapnies like on-one / cotic would be able to sort you out with a decent bike and offer you sizing advice etctoo.IMHO