You don’t always need the big gun. For a leaky toilet or a dripping sink, there’s a smaller valve right under the fixture. Look for a tiny handle behind the toilet or beneath the sink cabinet.
These are usually lever-style. Just turn them to the off position, and you’ve isolated the problem. This means you can still use water in the shower while you fix that toilet—luxury.
But remember: if the leak is before that little valve (on the pipe side), you’ll need the big main valve. That’s your failsafe.