If the water in the tank is cold and the air outside is humid, some of the humidity will condense onto the exterior surface of the tank. Think icewater in a glass on a picnic table in Florida - the glass "sweats".
The question is why this just started. If you lived somewhere else a year ago, it may have started because of the season and is normal for your new home.
If you've been there longer and it didn't happen at this time last year, either the air in your bath is now more humid or the water in the toilet tank is colder.
If you have a clothes dryer in your home, the exhaust hose may have developed a leak, putting a lot of moist air into the home. Boilng water on the stove raises the humidity, as will a humidifier or vaporizer. If the toilet is being flushed more often, the water in the tank will be colder. More frequent or longer showers will raise the humidity in the bath.
Replace a leaking clothes dryer exhaust. Toilet tank insulation kits are available at a good hardware store. A well-vented & efficient ceiling exhaust fan should be used during showers. You could also add a dehumidifier to the home, but that would increase your electric bill.
If one of the other posted answers hasn't solved your problem, and you haven't recently begun boarding a sorority girl or two (long, "Hollywood" showers and big hair), pose your question to a salesman (or two) at a good plumbing supply store (or two).
