reducing a 12volt fan speed
Speed Control: 
There are numerous ways to accomplish your task, it is a very common problem. To list a few solutions:


You could buy an off the shelf motor controller, there are very cheap ones all over the place like eBay and Amazon. Here is one that should work fine with your motor. Link Here 
If you like electronics, you could create your own PWM based speed controller using an Arduino or Analog 555 timer. This can be googled, as there are millions of examples of this, with tutorials far better then I can re-write here.


I would recommend option 1, and buy two control units, in-case one dies. Your fan is rated for 80W@12V, so any speed controller that supports at-least 12V, and at-least 150W (15A) should do the trick.

Wire:
As for the wire, the max power that fan will draw is 10A. if you are running it much slower, i would estimate ~3-5A, but always plan for the max amount of power to prevent fires etc.

for a 12V circuit running 10A, wire size should not be much of a problem. If your run is less then 10ft I would recommend a 18AWG or larger wire. Keep in mind, for AWG sizes, smaller number is thicker wire. Really anything in the range of 20AWG-14AWG would work fine, larger wire then 14AWG is just going to get heavy and hard to route.
Also I would recommend stranded wire, since it is more flexible and will handle setting up and taking down better over time.

Finally, for this low voltage wire, you can commonly find it at auto shops for cars, it is more flexible and thinner insulation then equivalent wire used for 120/240V houses.