Not enough info here. Is it the rim or the hub that is wobbly? If the rim is out of true (and not actually bent) the spokes can be tightened to bring it back into true. If the hub is loose then the bearings need to be adjusted. Either adjustment can be done by your local bike shop for $10-20. A broken spoke can also make the wheel go out of true, this will cost a little more to fix.
Answers & Comments
Not enough info here. Is it the rim or the hub that is wobbly? If the rim is out of true (and not actually bent) the spokes can be tightened to bring it back into true. If the hub is loose then the bearings need to be adjusted. Either adjustment can be done by your local bike shop for $10-20. A broken spoke can also make the wheel go out of true, this will cost a little more to fix.