Replacing Toilet Wax Ring Seal

If your toilet leaks water around the base/bottom, or rocks when you sit on it, or you smell sewer gasses (and there's water in the bowl) you may need to replace the wax ring that seals the toilet to the sewer pipe.

I'd replace all the cheap parts while doing this.

Flange/hold-down bolts, nuts and washers
The water supply line between the faucet-like "stop" at the wall and the toilet. I prefer the flexible braded metal ones.
The seal between the tank and the bowl if you separate them (makes it easier to lift and handle the toilet during removal and replacement).
And, of course, a new wax ring.

Read both of these how-to instructions before you begin:

Good discussion
Good illustrations