Well there are a number of ways to mount a light bar to the RZR. Our light bar mount is one way. Check it out when you get a second.
The next question is the electrical hookup. This is based on how you want to do it. If you want to hook up other accessories then get a power distribution block (with fuses) and run it from the battery to the 3 post connection, and from there run it to the power distribution block. We have a kit for that as well.
If you want the light bar only, then get enough wire to connect it straight to the battery. That or connect the 3 post to the battery and then connect that to the light bar. We do sell a 3 post only set of connections to the battery that's all preterminated and ready to install.
Now on the switch question, there should be one included with the bar. If there is not, you need to make a splice in the connection from the bar to the power connection. This is usually done with a relay. If you get a switch that is rated to handle the power the bar draws then you don't need a relay but usually they aren't something that would look good in a RZR dash. I would say since we could probably go into great detail about to make that work, I would just make sure to buy a bar that has a switch with it.
I hope this helps,
Al