Okay, explaining...

"dancing in the shower" is a little metaphor to describe that elated feeling you get when you're madly in love with someone...it's not a real phrase, but the whole song is about Alannis Morisette being madly in love in her relationship with this man until she reads the love note from another woman and realizing he's been cheating on her.

"smells like him" is not a phrase either...if you're ever in love, you'll notice that you're partner has a unique, wonderful smell that is particularly amazing to you. You ever hug someone and smell them and they smell good? Well, when you're in love that smell is enhanced and it's a smell you'll relate to with good feelings.