Put a knee to the edges of the commode bowl and gently try to shake it a little to see if it moves. If there is only a little movement (less that a ½-inch or so), and the commode doesn't rock up and down or sway side-to-side, you may be able to secure it by tightening the mounting nuts at the base of the commode. Tthese are usually covered by a removable porcelain cap. Use caution not to over-tighten them. If the commode lifts up or rocks more than ½-inch or so, you may need to replace the wax ring seal at its base. Unless you really know what you are doing, you should call a plumber at this point.