It's actually, "If the shoe fits". It used to be "if the cap fits", that was actually a popular way to imply someones stupidity or foolish behaviour a long time ago. "If the shoe fits" is now the phrase most people use today, and it basically still means the same thing, although we can still use "the cap" version if we intend to imply foolishness.

hello

hello

I think we usually say "if the shoe fits, wear it"