The basic wiring is simple- all you need are some scraps of wire and a powerpack. Attach the wires on the screw terminals marked:
"Variable DC: Track only"
and run the wires to the terminals on the track. You can buy terminal sections of track (they have screws for you to put the wires to) or you can get railjoiners with wire leads soldered to them. Place the treminal track section when you lay the track, or put the "wired joiners" in a place and drill holes beneath them. Slip the wires through the holes, and nail the track in place. Finally, connect the wires from the powerpack to the terminal or the wire leads.
You can use almost any type of wire, with gauge 18 to 22 being the most useful. I find that stranded wire is easier to work with than solid wire. If you layout is larger, you should consider running several sets of feeders around your layout, using a bus wire underneath to power them all. Make sure to get the polarity correct, or else you'll have a short. For the bus wire, regular house Romex (12 to 14 gauge) is certainly thick enough. Many modelers like to solder all their connections, including their railjoiners. This is not always necessary, and can in fact be a bad idea if you anticipate rebuilding your layout. If you do decide to solder, make sure you file away any excess solder from the inside of the rail so that your wheel flanges don't get caught.