Get a quality stripper, crimper, needle nose and terminal brush (what's a ratchet crimper? just don't use the generic multi-tool junk that strips/crimps/etc). Red electrical tape for easy ID of trail repairs and black for normal use. Rusty has a good summary above, except add a soldering iron and supplies. And pull off the red/blue/yellow plastic from the connectors (or buy them w/o); cover them with e-tape and heat shrink after crimping. Small terminal pick tools and/or tiny jeweler's screwdrivers are good to have.
Expect to pay about $25 for stripper and $30 for crimper pliers. The Snap On tool that Max posted might be good - it's much thicker than most multi-use models. Soldering iron can be be cheap at $10. Rosin core solder from an electrical supply place is better than from others. A fine - med fine solder will do well. I've got solder from many brands and places that just doesn't melt/bond right. Learn how to heat the part, not the solder.
Have a supply of good (cross linked polyethylene jacket) stranded copper wire in various colors and gauges (I keep 14g for most needs). Don't use A'zone/Wallymart-type vinyl coated wire if you can avoid it. Buy Mil-Spec battery terminals and install them now or keep them for when needed, just know that the bolt size might require you to enlarge the holes in any accessory ring terminals you have. A step drill bit or reamer is much easier than a regular drill bit for that.
Dielectric grease is good to have handy. You can get a small tube at A'zone. Also keep some 600+grit sandpaper in your kit for cleaning corroded contacts.
A multimeter of some sort is necessary. You can learn to use it as you go. Rusty is right - voltage and continuity are most commonly used. Add some alligator clip test leads for when you run out of length. Some people like a test light (has a wire to hook to ground and a positive test probe with a light to show voltage is present).
Check out
www.wirebarn.com for their GXL wire.
