The first "tool" you should obtain is knowledge. Get a good repair manual and/or visit the Park Tool's web site and sheldonbrown.com/harris and read up on various maintenance topics. If there is a bike co-op in your area, they are often an excellent source for hands-on instructions.

As to what to carry on the road, I agree that a set of tire levers. a spare tube (or 2) and a means of inflating it are the minimum along with the ability to use them. Beyond that a mini-tool with the common allen keys and, perhaps, a small chain tool will handle nearly everything you are likely to encounter on a ride.