the hardest part is deciding on your first bike......
Basically.... there are 2 main decision points:
Hard tail vs Full Suspension
26" vs. 29er.
(third is frame material).
Hard tails have a shock in the front fork and Full Suspension, as you would guess, have a shock/spring for the rear wheel. Full Suspension costs a lot more (not referring to cheap Wal-Mart, Dick's etc bike).
The bigger debate is 26" vs. 29er. Both have its pros and cons and followers of both types.
Here's an article to get you started
A popular bike last season on the 29er side was the Trek Marlin.
My bike, when I bought new a couple years ago.... cost $1000, with nearly the same equipment levels... so 29er prices gone down a lot....