What's a good bike for me?

Hey guys.
I've just started to get into riding a bit more, and I wanted to get a new bike. I'm looking for a mountain bike that will be used mainly on-road but will do a nice job as a trek bike.

I ride around fairlight/manly mostly (although I've only just started doing it regularly), so I want something that will feel good to ride during medium to long trips anywhere around the northern beaches.

I'm just on the border of 5"9 and 6 foot, and there should be some room to grow (I turned 16 last month).