how can i fix a leaky toilet?

Either the close coupled doughnut like seal has gone manky or the bolts holding the cistern to the pan have come loose. Go for loose bolts first see what happens.

To replace the doughnut cistern to pan seal the cistern has to be removed.
Turn off the water to cistern bail out the cistern or flush it. undo any wall screws at back of cistern inside.
undo removethe bolt wing nuts lift cistern clear discard the doughnut seal, clean up joint area on both pan and cistern. Replace cistern carefully not to dislodge the new seal. Seal, plate and bolts sold as pack in most DIY or plumbing stores.

Bob
May 2009