You may need to call a plumber to fix it. If the water shuts off, but continues to spill into the overflow tube, your float may need adjustment. In either case you will be wasting TONS of water and a lot of money until the problem is corrected. If you have one of the older toilets mentioned above, you may be able to save a substantial amount of water by placing weighted plastic bottles in the tank to displace and save an equal amount of water with each flush. The bathtub and shower are also major water users.
Baths generally use more water than showers.
Since at least part of this water that is going down the drain is HOT water, that you pay to heat, conservation is twice as important. Depending on your bathing habits, this can save you up to 10,000 gallons per year. Low flow showerheads should not be confused with the flow restrictors widely distributed several years ago. Flow restrictors provide a lower pressure, less pleasant shower. Even small leaks are expensive.