Its probably an easy fix. Most of the time, you can replace an old ballcock (fillvalve) with a Fluidmaster 400A. The installation is very easy. The valve comes with complete instructions for installation in the box. I would suggest starting with the replacement, its only about 10 dollars for the valve.
While your replacing the valve, purchase a new flexible stainless steel braided supply tube. It will save you tons of frustration, trying to reseat the old supply with the new valve.
Make sure you check the flapper inside the toilet as well, its perfect time for an overall maintenance of the tank.
Regular maintenance like this is something every homeowner should learn to save themselves hundreds of dollars every year in plumbing costs and damage.