How to fix leaking toilet?

The bolt that holds the cistern on to the toilet pan has a rubber washer. It could be corroded what is probably causing the leak

Toilet is very old I think this is happening because of rust u need to replace the parts or toilet

First you can try to tide ot up that bolt growing thrue cistern and is a washer on it so meaby that solve the issue

Check these first;
Remove the cistern lid and check water level. If the level is too high, adjust the Ballcock adjustment screw to reduce the amount of water holding in the cistern.
The 'Left white bolt' maybe the the water inlet. If this is leaking, then you might be better off getting a plumber to sort it out for you OR if you feel capable to do it yourself...
1) Turn off cold water feed into the cistern.
2) flush the cistern.
3) Undo the nut on the plastic 'white bolt' only half way, Dry the area around hole, and place "Plumbers Mate" around the hole.
4) Tighten the nut, and leave for about 30 mins.
5) Turn water back on and check if the leak has stopped.

There’s a donut shaped washer in side you should replace this if you ever split toilet apart