I’m 15 . On my left foot my pinkie toe, on the back has this white stuff under the skin?

"Back" of a toe doesn't mean anything. "Under the skin" doesn't mean anything. I suggest you go to images.google.com and search "white stuff on toes" and find an image that looks like what you are talking about. Then you can visit the site to get some knowledge about it.

In most cases, water and a toothbrush will keep your toenails looking just fine. Listerine and a toothbrush is even better.