How do I fix a constantly running toilet?

First open the tank and try to flush the toilet to see what is causing the continued running.  Make sure the chain is not tangled or trapped under the flow.  Look to see that the flow closes on its own after the tank drains.  If it doesn’t,  it may need replacing.  Check the floater;  if it’s not floating,  it’s probably full of water and needs replacing. If it still floats, bend the arm slightly to allow it to sit lower in the tank.