A leaky toilet tank can often be traced to the bolts that attach the tank to the toilet bowl. This is most often due to the bolts loosening over the course of time. Simply tightening the bolts is the most likely solution. Do not over-tighten, though, as this could cause the tank to crack.
Broken washers attached to the nuts and bolts may be the culprit, too. If this is the case, then simply remove the nuts and washers and replace the old, broken washers with new ones. Again, be sure not to over=tighten when replacing the parts to avoid causing damage to the toilet tank.