The usual problem in a situation like this is a bad wax ring.
The repair is a simple one. Turn off the water at the angle stop, flush the toilet holding the handle down till all water has drained from the tank. Disconnect the supply hose at the base of the tank. Remove the two cover caps on either side of the toilets base. Remove the nuts that hold the toilet down. Pull the toilet up and set it on some newsprint. There will still be water in the trap so have a bucket nearby to pour it into.
Using a putty knife remove the old wax ring. Tip the toilet and clean off any old wax ring from it also. Replace the toilet hold down bolts. They will most likely be rusted. Install the new wax ring and then very carefully set the toilet over the bolts and the new wax ring.
Using a rocking motion push the toilet into the wax ring till the toilet is touching the floor or at least very near to it.
Place the cover caps on the bolta and then the nuts and washers. Tighten both nuts semi snug. Now what i do here is stand on the rim of the toilet to get my full weight on it. Rock it with your feet all directions.
Once again tighten the bolts but this time snug them down good.
Put the water supply line on and turn the angle stop back on. Flush it a couple times and check for leaks at the base of the toilet. Apply a bead of caulking around the base of the unit and your finished.
For videos of how to do this just go to youtube.com and type in keywords "toilet wax ring repair. There are a bunch there. Some good most pretty cheezy.
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