Originally Posted by
ljsense
Get a set of metric allen wrenches, a set of tire levers, and a can of lube.
Originally Posted by
aggiegrads
A floor pump if you don't already have one.
Start with these, then pick up other things if/as you need them. I'd add a chain tool to the beginning list.
Most of the time it's worth getting quality tools (Park, Pedros, etc.). If it's truly going to be a one-off job, it's often cheaper to let the mechanic at your LBS do it than to buy the tool yourself. (It's also a good way to build a relationship there, so when you walk in the eyes of the mechanics all light up and they run to see what "interesting" problem you've got this time.)
Some things, like wheel truing, take a bit to buy all the tools you may need -- spoke wrench, truing stand, tensiometer, not to mention "The Bicycle Wheel" by Brandt so you know what to do with it all. (Yes it's possible to do without all those except the spoke wrench. Nevertheless, once you've got them, you'll see the value.) But it's not hard, once you've collected everything and fixed a few wheels, to do a better job than most mechanics.