The 10-minute diagnosis anyone can do
Lift the tank lid and set it somewhere safe, flat, and padded, because they crack easy. Now look and listen. If the water level is creeping over the top of the vertical overflow tube in the middle of the tank, your fill valve isn't shutting off: that's culprit one. If the water level sits below the overflow tube but the toilet still runs or randomly refills every so often, water is escaping past the flapper at the bottom of the tank: culprit two. Not sure? Put five or six drops of food coloring in the tank and wait twenty minutes without flushing. Color in the bowl means the flapper is leaking. That one test settles most of it.
The two fixes worth doing yourself
The flapper: turn off the little shut-off valve behind the toilet, flush to empty the tank, unhook the old flapper from its pegs and chain, and take it with you to the hardware store so you buy the matching style. Hook the new one on, set the chain with just a little slack, turn the water back on, done. Ten minutes, no tools beyond your hands. The fill valve: many just need the float adjusted so the water stops an inch below the overflow tube, usually a screw or a slide clip on the valve. If the valve is old, hissing, or won't hold its setting, replacement valves are inexpensive, come with instructions in the box, and connect with the same two skills: shut the water off, don't overtighten plastic nuts. If you can assemble furniture, you can do this.
When to stop and hand it to someone
Here's where honesty pays. Call for help when the shut-off valve behind the toilet won't turn or drips when you touch it, because forcing a corroded valve can turn a five-dollar problem into a wet floor. Call when you've replaced the flapper and it still runs, since that can mean the flush valve seat itself is pitted, which is a bigger job. Call when the toilet rocks, the floor around it feels soft, or you see water at the base, because that's a wax seal or subfloor conversation, not a tank part. And call if you just don't want to deal with it, which is a completely legitimate reason. Ken handles all of the above at the fixture, tells you straight if anything crosses into licensed plumber territory, and doesn't make you feel dumb for the chain being tangled.
What it costs you to ignore it
A running toilet isn't just an annoying sound. A moderate leak wastes hundreds of gallons a day, and Water Services bills in Kansas City are climbing every year. People discover a running toilet through a water bill jump all the time, so if your bill spiked and you can't figure out why, go do the food coloring test tonight. The fix, DIY or done for you, typically pays for itself inside a billing cycle or two.