1980's young adult book, begins with girl using shampoo in shower and imagining it will change her l

For some reason I have a strong memory of reading this book and I want to get it for my son. All I remember about it though is the opening chapter of the main character showering with a shampoo and imagining that when she was done she would look like the girl on the shampoo container. If I remember correctly it might have been science fiction/fantasy, but I could be wrong about that

This is a long shot, but could it be Desdemona - Twelve Going on Desperate? One of the professional reviews mentions a shampoo mishap, but it doesn't sound quite like what you've described.

Tough Luck Karen by Johanna Hurwitz also begins with a shampoo mishap. But it's not sci-fi/fantasy. It's about a girl who cooks and baby-sits.

It sounds like The Changeover by Margaret Mahy. That is the opening scene, and it is a fantasy.

The Changeover immediately leapt to mind for me too!