Newby building a motorised board
If you are thinking of building from scratch (i.e. sourcing/making and integrating the battery, controller, control logic, firmware, board, wheels, drive chain), this is probably a larger project that you first expected.

Just to back up my point, I followed the same route, and after 550 hours and NZ$3,500 later (yes, 550 man hours!) came up with this...http://cladlab.com/electronics/projects/electric-skateboard/electric-skateboard-design



This isn't an advertisement (not that I make any money from it), but hopefully you can glean some information from that project page. In summary, I used an 800W brushed DC motor (you could go for a brushless one, slightly more expensive, and more complex control but better power-to-weight and size ratios), which fitted onto a custom axle, drive chain, and wheels, all designed for an electric skateboard already. Have a look on eBay for these. Doing this yourself would add significant time and resources (e.g. requiring a lathe/milling machine). 

One motor will be sufficient, as long as it is powerful enough (one-wheel drive works surprisingly well, as mine is). Two motors means two drive chains, possibly two controllers, and stuff like differential control for steering e.t.c.

Also, power electronics are notoriously difficult to design, and I definitely recommend buying a motor drive module. You can get cheap ones like the one shown in the image below off http://www.aliexpress.com/ (and here is a link to one to get you started).



As for the control logic, this is something I recommend you do yourself, and you have the right idea by using an Arduino. 

As mentioned already, probably the best way to start is to modify something that already moves (like an electric wheelchair). An interesting project, and I wish you all the best.