Houses
In most Toronto houses the water service comes in through the basement wall on the street side, and the main shutoff is right there, next to the water meter. Often there is a valve on each side of the meter. The one on the house side is the one to use.
Older houses may have a round handle you turn clockwise. Newer ones often have a lever: when the lever is across the pipe, the water is off. If the basement is finished, the meter and valve are usually behind a small access panel or in the furnace room.
Condos and apartments
A condo unit usually does not have one main shutoff the way a house does. There may be separate valves for the kitchen, bathroom and laundry, often under the sinks, behind an access panel near the washer, or inside a utility closet. Some buildings only shut water off from the mechanical room.
Ask your building management where the valves for your unit are before anything goes wrong, and keep the concierge or superintendent's number in your phone.
Test it now, not during a leak
Old valves seize when they are never used. Turn it off, check that a tap upstairs stops, then turn it back on slowly. If it will not move by hand, do not force it with a wrench. A valve that breaks open is worse than one that is stuck, and replacing a seized main valve is a small job when nothing is flooding.
Tag it
Hang a tag or tie a bright cable tie on the valve and tell everyone in the house where it is. In a leak, the person who finds it is often not the person who knows where the shutoff is.