If you are noticing that your toilet's tank isn't filling up properly or there is little water flowing into it when you flush, then the siphon valve--or fill valve, as it is better known--needs to be replaced. This controls the flow of water into your tank from your home's plumbing. Hardware stores carry universal replacement siphon valves.
Close the shutoff valve behind your toilet.
Flush you toilet, emptying most of the water out of your tank.
Unscrew and remove the supply line from the toilet running to the shutoff valve using a pair of channel locks.
Unscrew and remove the white plastic locking nut on the underside of your toilet tank, holding the siphon valve in place. Remove the old siphon valve.
Set the universal replacement siphon valve in the hole in the tank.
Set the valve's gasket between the base of the valve and the tank.
Place the black washer over the threads on the end of the siphon valve sticking out of the bottom of the toilet tank. Screw on the white plastic locking nut.
Set the small hose running from the top of your new siphon valve into the small opening on the top of your flush valve.
Open the shutoff valve and flush the toilet, making sure the valve is installed properly.