The most common culprit for unwanted water usage is found in the toilet tank, which is found at the back of the toilet under the porcelain lid. There you will find a toilet fill valve, which automatically allows clean water in to until a set shut off point to serve as the next flushing water. Also, you will find the flapper and flush valve which actuate the flushing action and controls water the water level in the tank. If you hear a hissing sound that does not stop when the tank should be full, you likely need to address the fill valve. If you see water rippling in the toilet bowl when the toilet is filled (often this can happen with no detectable sound of water running), that means that you have a toilet flapper that is not seating properly.
To fix the toilet fill valve, you have two choices: 1. You can try to turn off the water valve running to the bottom of the toilet tank (usually on the left side as you face the toilet), open the top of the valve carefully and see if there are bits or rust or other debris that may be keeping the valve from shutting off or 2. Get to your local Hardware store and buy a new valve assembly to replace the one that is not working properly. If you do not have too much experience but some amount of curiosity about it, explore the first option and if you find you can not figure it out, don’t worry about it and go buy a new one. Fluidmaster makes a nice and economical solution that does away with the older float type of valve and comes with great instructions. They should be less than $10 at your local hardware store.
The water that flows into the tank is clean water (the same that comes to your kitchen tap, unless you are using a grey water system to fill your toilet). If you are replacing the toilet valve, you will need to turn off the valve to the toilet and then flush the toilet once to void as much water as possible out of the tank. Then, unscrew first the hose running from the shut off valve to the toilet valve and then remove the valve from the toilet tank, a pair of channel locks is a good tool to help you get the nut on the bottom of the toilet tank loose. An adjustable wrench or pliers will also suffice if you don’t have or want to buy channel locks. Have a small cup and a towel ready on the floor underneath that valve when you remove it as water will spill out that did not get flushed out earlier. It is not a lot of water but enough that you will want a rag to soak it up.
If you suspect the problem is the flapper on the flush valve, turn off the toilet, remove the rubber flapper and take it to the hardware store to get something compatible for a replacement. These flapper are just rubber and they all wear out after a few years.
There are some less than common toilets out there and if you have one you probably know it. If the toilet is a low tank type or unusual in other ways, you may need to call the company that made the toilet or a more specialized plumbing distributor for replacement parts.
Remember, if your toilet has a leak, it is wasting a lot of water and even if you cannot fix it, have a plumber do it promptly.
While you are working on the toilet, make sure the toilet bolts are snuggly holding the toilet to the floor and apply a bead of silicone sealant around the entire toilet base where it meets the floor to help reduce draft from the whole in the floor under the toilet. Make sure the floor is dry before applying the caulk.