Metaphor and Metonymy? Polysemy and Homonymy?

I'm doing a linguistics assignment but there are several problems:

1. I've never studied linguistics

2. I can't seem to differentiate between what a metaphor and a metonym is....which may be because

3. Everything needs to be in French and my questions are in French.

I need to differentiate between which definitions of the word 'feuille' (leaf in English) have come about to to metaphor and which have developed due to metonymy.

So PLEASE, explain like I'm five, what the hell is the difference?

Thanks in advance!