Must-have tools. . . Let's see:
I'd start out with a mechanic's tool set from Sears like
this
More sets of mechanics' tools here
You'll want to make sure you have regular and deep sockets, English and metric, and in 1/4", 3/8" and 1/2" drive (that turns out to be a lot of sockets, but believe me, they come in handy)
Sometimes you can get a set with a plastic molded case included, and that all fine and good, but a real tool box is better. Ratchet extensions and universal joints also come in VERY handy, I would make sure to get some of those in varying sizes.
Get yourself a big ol' set of screwdrivers, too. Oversized ones are also useful for prying. Let's see, a couple sets of vice grip pliers, 2 large C-clamps. . .
These brake tools can make your life quite a bit easier
A Ball Pein hammer, spark plug gauge, a set of torx head sockets, tire pressure gauge, soldering iron, wire strippers, a big ol' set of zip ties, an oil filter wrench that fits your car's filter (or a strap-wrench), a set of pliers that includes a couple sizes of channel-locks, an automotive circuit tester, a multi-meter. . .
What did I miss, fellas? I like to use Craftsman tools 'cause when you break 'em, you can just wander into any Sears store and they'll just hand you a replacement. Having Sears stores all over the place makes it real convenient. If you get el cheapo tools they'll rust up and get all nasty, and sometimes just plain break into pieces.