If you have 2-part toilet, then tank is attached to bowl with 2 bolts.
Look inside tank and you can see bolts with rubber washers located on either side of flush valve.
The 2 bolts hold tank and bowl together and between tank and bowl is a washer called the spud washer.
So spud washer could be leaking ... or 2 bolts could be leaking.
1) If bolts are leaking, there is usually dripping all the time, and drips are visible on bottoms of bolts
If spud washer is leaking, then tank could leak only when tank is flushed.
2) First thing to try is tightening nuts on the 2 bolts. Nuts may be loose. Tighten nuts a few turns, and check if toilet still leaks.
3) To fix a spud washer, you usually need a new generic one from hardware store.
Newer tanks with 1.6 gallon flush have special sized spud washer.
Tank is upside down and spud washer is fit over flush valve
If you cannot find new spud washer, re-use old one and smear retorseal on it during re-assembly to make sure it seals. Rectoseal is good for many home plumbing repairs. More about that in a moment.
4) To take apart tank: Disconnect supply line to tank. Empty water out of tank. Remove nuts from bolts. Lift tank off. Wash out tank with hose. Make sure there is no sand or sediment left in bottom that might clog up flush holes located inside bowl after tank is re-assembled.
5) Remove old spud washer, and put new one on. Do a google-image search for 'spud washer' if you need more photos. New spud washer fits over flush valve. That's it. And then reassemble tank.
If you want to apply rectorseal to washer, put washer on as shown in photo above. Then smear-or-gawm rectorseal over area of washer that sits on bowl. Rectorseal will make sure there is no more leaking.