With more details we could probably tell you down to a component level what is causing the problem. But as Kok328 stated replace the internal tank components and the supply line from the angle stop valve to the tank as well.
Generally most toilets will take a Fluidmaster 400A Fill Valve, a Korky 2001BP or, 16BP flapper and a 12" braided stainless steel supply line. (Avoid Watts FloodSafe Supply Lines)
However, some toilets may require other proprietary parts as well. Can you tell us what toilet you have or, supply a picture of the inside of the tank?