A toilet that runs continuously can waste well over two hundred gallons a day. On a California water bill that shows up fast, and the constant trickle also wears the flapper seat further, so the problem accelerates. The good news is that the three most common causes are all inexpensive parts.
Take the tank lid off and drop a few drops of food coloring into the tank water. Do not flush. Wait fifteen minutes. If color appears in the bowl, water is leaking past the flapper at the bottom of the tank, and the flapper is your problem. If the bowl stays clear but the tank keeps filling, the fill valve or float is the culprit instead.
Rubber flappers harden and warp with age, and chlorine tablets dropped in the tank destroy them quickly. Check that the flapper sits flat on its seat and that the chain has a little slack. A chain pulled tight holds the flapper open a fraction, which is enough to keep water moving. If the rubber feels stiff or gritty, replace it.
If the tank overfills and water pours down the overflow tube, the fill valve is not closing. Sometimes lowering the float adjustment fixes it, since the water level should sit about an inch below the top of the overflow tube. If adjusting the float changes nothing, the valve itself has failed and needs replacing.
Less common, but a hairline crack in the overflow tube lets water drain into the bowl continuously no matter what the flapper does. This one usually means replacing the whole flush valve assembly, which is a job most homeowners hand off.
If you have replaced the flapper and the fill valve and it still runs, the flush valve seat is probably pitted or the tank has a hairline crack. At that point the repair cost starts approaching the cost of a new toilet, and it is worth getting both numbers before spending more on parts.
A real plumber, a flat-rate quote, any hour of the day.