What type of riding you wanting to do? Are you going out with people who ride already or just starting on your own?
You could either get a cross country bike (light and full suspension or hardtail). Suited for those long rides out along long trails and doing jumps and drops aren't recommended on these.
A all mountain bike (usually full suspension and a bit heaver with stronger components and shocks that will take more of a beating).
In my opinion the best type of bike to get. You'll get to do a bit of everything on it and can also take on long rides and do your jumps and drops.
Or Downhill which a person starting out won't get straight into and an all mountain bike will do downhill too but not the extreme stuff.
If you like bombing hills fast and doing big gaps and big drops then you'll find you would want one of these bikes. But you're starting out so you wont be needing one at the moment.
Decent Cross country and All mountain bikes can be picked up second hand for around $800 if you're lucky to $2000 and usually they sit around the $1500 price.
Try stick with full suspension rather than hardtails for a smoother ride.
You'll be suited to a medium frame by the sounds of your height.
But yeah I'm sure someone would be able to help you out even more. Or you could post at
www.vorb.co.nz