Every toilet and most sinks have a small oval valve on the wall or floor below them — turn it clockwise (righty-tighty) to stop water to just that fixture. Your home also has a main shutoff (often near the water heater or where the line enters). Find them now, so a leak is a two-second fix, not a flood.
Use a flanged plunger — the kind with the rubber sleeve that pulls out. Get a good seal and plunge firmly a dozen times. If the bowl is near overflowing, take the tank lid off and push the flapper closed to stop more water first.
Take the tank lid off and look. Usually the rubber flapper at the bottom isn't sealing — jiggle the handle, or press the flapper down by hand. If the chain is tangled or too long, straighten it so the flapper closes fully.
Pull out any hair or gunk at the stopper. Try a plunger, or a cup of baking soda followed by hot water. Skip harsh chemical drain openers — they can damage pipes and rarely fix the real clog.
Unscrew the aerator (the little screen at the tip of the faucet) and rinse out the grit, then screw it back on. Mineral buildup there is a common cause.
Close the shutoff valve for that fixture (or the main), put a towel and bucket under it, and send a work order. Stopping the water buys us time to fix it right.
Water is actively flooding, coming up through a drain, or you can't stop it at a valve — shut the main off if you can and call us right away; this one can't wait.
Sorted — nice work. If a drain keeps slowing down or a toilet keeps running after you've reseated the flapper, put in a work order so we can clear or repair it properly.
Tell us which fixture, and whether you were able to shut the water off — a leak with the water stopped is routine; one still running is urgent.