I went to Wal Mart (other stores have similar kits) and for less than $10, I got one heck of a nice tool kit. Pliers, wire cutters, standard and metric sockets, standard and phillips screwdriver etc... In addition to this, I keep a little bag with small and medium hose clamps, spare fuses and plugs, my plug wrench and a few "necessary" items that will always bail you out, electrical tape and wire ties. As for the tools, the have a nice box that keeps everything in it place, it stores "flat" so it doesn't take up space, and it seals rather well to keep out moisture. You also don't feel too badly if you drop one overboard ore loose something out of the kit.
One last thing that some people have trouble with, make sure that you have a lug wrench that will work on you trailer wheel lugs. I ran into this situation. I now carry a four way wrench in both of my vehicles.