Hi Tammy Condensation on a toilet tank is due to excess moisture in the air (often occuring in basements) condensing onto the cold toilet tank. A few possible ways to counter this. First, if its in a basement, run a dehumidifier to remove room air moisture. Second, cut some rigid insulation and fit it tightly around the inside of the tank (after you have drained the tank, of course), sealing joints with silicone that you let dry before refilling the tank. Third, wrap the outside of the tank with with one of those decorative fabric tank covers, although they often don't cover the bottom or back of the tank. Finally, you can run a hot water line over to the water inlet line with a 'T' and mix hot water with the line cold water to warm up the water going into the tank.
When I finished my basement, I had a similar problem, which I resolved by using a basement dehumidifier.