If there is just a cm or so of wobble you need to re-tension the spokes using a spoke wrench.
You can retension on the bike, using the brakes as guides to trueness. It is a delicate skill and requires small turns of the spoke wrench (1/8 to 1/16 of a rotation).
If there is major taco-style wobble, you can do a get-you-home repair but basically, the wheel is shot and you need a new rim.
For a "quick fix" find a big tree, hold opposite sides of the rim that are closer to you, put the axle on the tree and give the wheel a shove toward the tree. This may be good enough to get the bike rolling again but the wheel will still need to be trued, re-tensioned, and re-dished. And the wheel may need a new rim and spokes. A tacoed wheel is usually the sign of insufficient spoke tension.
Yep what MichaelW said. I have a spoke wrench, and even take it on my rides just in case the rear wheel gets a bit buckled. Its a cheap option, and will save you money instead of getting the bike shop to do it. Bikes with 'V-Brakes' will notice and are sensitive to small buckles due to the pad rubbing so a spoke wrench is a nice little tool to have on you.
This is mine:
This nifty tool is only 5cm in length and I obtained it for just $15AUD from my LBS.
Parktool have an excellent description to follow for truing the wheel here.
You can do this buy keeping the wheel on the bike, just take the tyre and tube off first, and if you have a 'v-brake' setup, you can easily spot out the 'un-true' parts.