I take it it's a close coupled one ie: cistern sits on top of pan if so it's likely to be the thick sealing washer between the cistern and bowl. Turn water feed to cistern off and flush it. Release the water feed from the cistern.
Undo the two screws inside at back of cistern that fixes it to the wall[if it is fixed].
Undo the pan to cistern fixing bolts remove the cistern replace the thick washer obtainable at plumbing outlets or the DIY stores. If the cistern is not fixed to the wall via the rear top screw holes then fix it as movement can cause it to leak again.
Obviously reverse proceedure for re-fixing.
Rob
July 2008