i can never tell the difference between an idiom and a metaphor. is there one?

He is pushing up daisies (i.e. dead)

Idiom = not the literal meaning)

See 'idiom' in the Grammar Monster glossary of terms

He is a star.

Metaphor = stating something is that it is literally not.

See 'metaphor' in the Grammar Monster glossary of terms

Excuse my typos - you get the idea.