my english teacher asked us to find an Idiom that english people use (don't know if its British or American).

The concept of it is about when someone tries to act like a hero, but stupidly. i don't how should i explain it, like when someone tries to do something big, like saving someone's life, but they use a stupid way. They themselves think that they've done it the best way, but actually everone else think its stupid.

sorry i don't know how to explain it, hope you'll understand.