Clogged drain: A drain cleaner may be used, but make sure there is no acetone in the product. Acetone can rot out cast iron piping and the glue on plastic pipes causing leakage. Preferably have the pipes rodded. CPC can refer you to a quality sewer and drain cleaning company.
Clogged toilet: Plunging is not recommended by CPC. You can risk leakage from breaking the seal on drains or breaking the seal to the toilet. Call a professional plumber to auger the toilet.
Jammed disposal: To reset disposal if it is humming, push the red reset button of the disposal, located on the bottom underneath the unit, or use an allen wrench in center of disposal to unjam if no reset button is present. Next, check to make sure the power is on. If the power is on and the unit is still non operational call a professional plumbing company.
Leaky valve (i.e shut off valves, boiler drains, etc.): On almost every valve there is a packing nut (this is a nut located at the back of the valve). By opening and closing the valve the packing nut may become loose, needing to be tightened. Tightening the packing nut will stop the dripping in most cases.
Unstable toilet: The seal is probably broken and this will cause leakage. Call a professional plumbing company.