How do I fix a leaky toilet that only leaks from underneath?
You can try replacing the wax ring.  Usually, that will fix the type of leak you describe.

Essentially, there is a wax gasket which seals the area between the flange (in the floor) and the porcelain (on the bowl).  Due to age/movement/etc, this gasket will eventually fail, and cause leaking when the toilet is flushed.

Replacing the ring is a relatively straight forward procedure:


Remove the toilet

Turn off the water to the bowl
Disconnect the flexible water pipe from the shutoff valve
Flush the toilet, holding down the handle to get rid of as much water as possible (use a plunger to push the remainder out)
Loosen the bolts on either side of the bowl (These may be corroded or completely stuck.  You may have to hacksaw these off)
Gently rock the bowl side to side to release the seal, and then move the toilet off of the drain
Invert the bowl, exposing the seal on the bottom of the toilet.


Install a new wax ring

Remove the old wax material from both the bowl and the flange with a putty knife (you will likely also find a plastic "funnel", which can be removed as well)
Take your new wax ring, and install it on the bottom of the bowl (the rounded portion will be facing up towards the bowl.
If you were forced to hacksaw the bolts from the flange, install the replacement bolts to the same position on the flange.


Reinstall the toilet

Lift the toilet back up, and thread the bolts through the holes on the side of the bowl (do not let the wax ring touch the ground until you are ready to drop it in its final position).
When the bowl is in its final resting position, sit on the bowl to seat it in place.  Rock back and forth gently to squeeze the wax ring.  The base of the toilet should now be resting on the ground around the entire base.
Reinstall the washer and bolts holding down the toilet.  Be careful when tightening these bolts (do a little at a time, switching between the 2 as you go).  You do not want to break the flange or the bowl.
Trim up the bolts with a hacksaw to adjust the length (if necessary). Reinstall the plastic caps over the bolts.
Reconnect the flexible water line to the shutoff valve
Turn on the water