Take off the lid to the toilet tank and look inside. You should see a flapper make sure it is resting on the bottom of the tank.
Also you can clean off in between the flapper and the tank.
And make sure the chain that holds the flapper is not caught on something.
If this dose not work you will need a new flaper.
Hello. One more thing you can replace the fill valve cap this will stop the toilet from going on and off all the time.
Here is the part you need.
This replaces the rubber seal
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If this is not what you have then you can just replace the fill valve they do not coast much.
Part # XXXXX 528T
To replace the valve first turn off the water then flush the toilet to get the water out off the tank then use a sponge to get the rest of the water out . Then remove the fill valve and replace it with this one above.
That will fix your problem.