More than likely it’s the gasket between the tank and the toilet. Here are your options for leaks on a toilet,
1) Check water supply line from stop valve to toilet.
2) Check Gasket between tank and toilet, water will track to the floor so make sure you follow the leak to the source. To fix simply shut off water (should have a stop valve near toilet) and flush toilet to remove water in the tank and make sure the supply shut off is holding. If you have a flex connector with excess line you shouldn’t have to remove the water supply line to the toilet. If your water line is hard piped simply remove the line from the toilet so you can take off the tank. Next remove the tank from the toilet, to do this if you take the cover off the tank there should be a couple of screws with rubber gaskets under them that need to be removed, on the bottom side of the toilet the screws will be backed with some plastic nuts that the screws insert to, make sure they are not spinning on you when you loosen screws. Replace gasket and repeat steps for assembly. You will need a cresent wrench, screw driver, pliers and a new gasket. You are working with porcelain so don’t over tighten. If the tank continues to leak you probably have a crack in the tank.
3) Check the base of the toilet, if water is leaking when you flush, replace the wax ring.