If you're talking about a leak when the water is turned off - then that probably means something in the faucet is allowing water to flow in spite of it being off/closed.

You have to turn off all of the water to the home because you probably don't have a little valve to just turn off the shower like you have on a toilet or regular faucet at a sink. That's the only reason it's "whole home" versus just the little valve under the sink.

But the concept as to what's wrong will be similar to a sink faucet. Might need a new cartridge. Might need just a washer - depends on the make/model.

I'd start with a youtube search. Chances are you'll find step by step instructions, and it isn't that tough to fix.