How do I fix a leaking toilet ?

You can give this a try and see if it helps you out.
On the other hand, you can search/have a look around the back side of your toilet wall (outside) for the water inlet/supply pipe; there must be a control on it.

1. Turn off the water at the shutoff valve, flush the toilet and hold down the handle. There may be a little water left in the bottom of the tank, but don't worry about it. Wipe the flapper seat with a clean cloth to make sure it's free of debris, and check it for cracks or splits (Image 1).

   How to Fix a Leaking Toilet ' how-tos ' DIY

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How to Fix a Leaky or Runny Toilet - This Old House

www.thisoldhouse.com > ... > Toilets

This Old HouseA toilet that leaks or runs is an easy homeowner fix-if you know how to diagnose the trouble. ID parts and problems with this handy guide, and hit the hardware ...

The wax ring seal on the bottom of the toilet has let go. It isn't tough to fix. Two nuts hold the toilet down. Shut the water line in back off, take the 2 nuts off, and move the toilet aside. You;ll need a new wax ring, so you'll have to scrape the old one off. It will show you how to set it in and set the seat on. It only looks tough, but it's actually simple. Good luck!

Leaking to drain? Adjust float height to keep water below overflow ... or change flapper if it doesn't seal.

Leaking to floor? Check either end of supply line; or check seal between tank and bowl.

If the leak is inside the toilet, then you have a defective flush valve or flapper Which ever one is in your model toilet.

1. I changed one in my 76 recently. Loosen bolts on the side towards the rear of toilet. remove water line. (shutoff first) pull straight up to remove. I t is pretty easy.
   

Sounds like the washer in the shutoff at the wall has failed and is partially blocking the fill pipe. Some rubber from it may have gotten into the toilet valve internally. With the wall valve closed, try to remove the connection pipe to the toilet. If you can do it without water continuing to flow out of the wall shut off...then remove the pipe from the wall shut off. Flush the toilet. Now, apply air pressure to the rubber hose the goes into the overflow pipe. That may clear the valve in the tank and solve both problems. If you cannot stop the water at the wall shut off, close the water valve feeding the house. Remove static water pressure by opening the cold water a the kitchen sink faucet. Now you can open the wall shutoff valve and replace the washer and clean it out....

Some shutoff valves are pretty marginal to start with, then they corrode. They can easily break if you put too much force on the handle. You are wise to be cautious. You will have to turn of the main water supply and then replace the toilet shutoff valve. You might be able to replace the shutoff valve yourself, depending on your skills and depending on how the shutoff valve is connected to pipe coming out of the wall or floor. But keep in mind that if you get in over your head you will have no water in the house until you get that new shutoff valve in place.
If you can describe what kind of pipe that shutoff valve is connected to I can help you understand if you should tackle it yourself.

There's a lot of variety in the works in the toilet tank. Assuming this is not a pressure-assisted toilet-----First understand the three valves are: 1. The shutoff valve - on the supply line coming into the toilet tank. 2. The fill valve - inside the tank, opens with the float descends, closes when the tank fills up. 3. The flush valve - inside the tank on the bottom. Common old style is a flapper on a pivot. Yours may operate straight up and down rather than on a pivot.
Three very common problems are---- 1. The chain that connects the handle to lift the flush valve is getting caught, not allowing the flush valve to close. 2. There is problem with the flush valve being able to get a good seal when it closes - could be debris or misalignment. 3. The fill valve is not closing when the tank is full, could need adjustment or there is debris preventing the valve to seat. . Here's are some steps to diagnose---- close the shutoff valve. Wait a few minutes as you watch the level in the tank. If the water level decreases, you have problem with the flush valve. To confirm the fill valve as the problem, turn the shutoff valve back on. Lift up on the float that is supposed to turn off the fill valve. If it shuts off, you should be able to adjust it - most have a +/- plastic adjustment screw. But if raising the float by hand only slows it down, you have problem with the seating of the fill valve - which can sometimes be cleaned, but more often would need to be replaced.
I hope this is helpful. Thanks for using FixYa.

Your ball **** needs replacement (water control valve), Also your flapper or tilt valve disk is leaking water through it and it needs replacement also.