It's not the rims you need to change, it's the hubs. Although it is a common mistake to refer to the whole wheel as a rim, so I assume that's what's happened here.

Does the bike currently have V-Brakes on it? If so, that's pretty easy, as you can use the same lever for most cable disc brakes.

It's relatively straight forward. Install the fork, and the brake rotor on to the hub. Fit the wheel. Bolt the brake caliper onto the tabs or posts of the fork (depending on the style of mount)

Then attach the new brake cable (You'll need a new inner and outer cable) and adjust the brake. There's plenty of tutorials online about how to adjust them.

Give this artice a read.

http://www.parktool.com/repair/readhowto.asp?id=124