Usually when a toilet runs it's because the flapper is worn out. You can get a new one and replace it fairly simply. Hopefully that's the part that's leaking, it will be inexpensive fix if so.
I suggest that you take off the tank lid, down where the water goes out when you flush therer is a seal. Turn off the water supply to the tank. There could be some grit so just lift the "FLUSH" handle and hold it up now run your fingers gentle under it and to wipe it. Turn on the water again and see if it worked.
https://www.fluidmaster.com/toilet-problems/identify-fix-toilet-flapper-leaks/ I have used fluidmaster for all repairs to the toilet. You can buy them at a home improvement store.