Hey Gus,
Best advice anyone can give you is to find your nearest shop/dealer and try and demo as much as you can. (Check out the demo fleet at wind surf and snow at collaroy or join us at Jamison Park Paddle for the regular Sunday morning TT where there is also a large demo selection and lots of newbies)
In my opinion go straight for a 14. At your weight and height you could go reasonably narrow, say 25" - 27".
The stability in some of the newer 25" wide boards now is pretty solid. You may be pleasantly surprised at just how narrow you can comfortably go.
Again though, demo, demo, demo.
Post your progress here for some feedback.

Good luck!!