Step 1: Nobody wants to waste water and your toilet flushing non-stop isn't going to help. To fix it you first need to locate the problem area by removing the cistern lid. Flush the toilet when the cistern lid is removed and check each component to see if there is something that obviously isn't right.
Step 2: Is the water inside the cistern sitting below the water line? If it is then check if the water valve is fully on. If it isn't simply adjust the valve and water will fill up the tank. If this does not work then the problem may be something else.
Step 3: The most common cause of a running toilet is that the flapper is stuck open and not covering the valve. This is a fairly simple fix, just reach into the cistern and close the flapper, Check the chain attached to the flapper as it could be tangled or twisted preventing it from closing. If tangled then simply untangle the chain using your hand. If the flapper still isn't closing then there may be an issue with the part itself and will need replacing.
Step 4: If your toilet is still running then the next part to check is the float. Lower the float to lower the water in the cistern. This should stop it from rising too high. When the water level is too high, the water goes through the overflow pipe into the toilet bowl. This is what causes the toilet to keep running. The water level should be no higher than an inch below the overflow pipe. If your toilet has a ball float, bend it downwards to the level you want the water to be. If your toilet has a float cup, find the metal clip and then pinch and slide it so it is beneath the water level.
Step 6: Check the float to make sure that it isn't clogged with water. This can cause the toilet to run. To check, unscrew the float and gently shake it, if you hear water inside then replace the float with a new one.
Step 7: After checking everything we have ran you through, leave the cistern lid off and flush the toilet to check that it has stopped continuously running. If it still runs, the flush valve or flapper may need replacing. If it doesn't fun you have fixed it. Well done!