We can probably talk you through this.
First thing we need to do is make sure of what your problem is.
Shut the valve under the toilet off fully. Flush the toilet, and hold the handle down until all of the water empties. Use a shop vac or a sponge to get most of the rest of it. Then, if the tank slowly starts refilling with water, you know that the shutoff valve isn't holding.
Next, add some food coloring to the tank, and then turn the water back on. Make sure that there is enough food coloring to give a decent tint to the water. Now you will be able to tell for certain if the leak is coming from the tank or the supply line.
Report back, with a few pictures.
Lake County, Florida plumber
Springtime Plumbing Services