Look to see if the chain or connection to the float arm has become disconnected. If it has, refasten it. If it is broken, it will need to be replaced.
Jiggle the float arm. Sometimes that is all that needs to be done to get the water to stop running.
Bend the float arm downwards slightly, if the water continues to run.
Check the float ball, if there is one, to see if it is full of water. If it is, it will need to be replaced.
Squeeze the clips and slide them down, if you have a float-cup assembly float system.
Adjust the pressure-adjusting screw on the valve, if it has one. This will be located on the valve on the bottom of the tank.