If your toilet is running and/or leaking, you can easily find out what's wrong with your toilet by putting some food dye in the water in the tank. Once the water has the dye, you then need to look and see where the water shows up.

If the colored water shows up in the bowl of the toiler, your flapper valve is leaking (it's the big rubber thing in the bottom of the tank that stores the water).

If the colored water is running down the outside of your toilet, you need to replace the seal between the toilet and the tank.

If after a couple of hours you still see no colored water, try flushing the toilet twice. If you then see colored water on the floor with none on the toilet, you need to replace the wax seal between the toilet and the floor.