Yesterday I stepped on a little nail? Probably went in less than 1/4 inch, and looks fine now. Should I get a tetanus booster?

Depends. The depth of the injury means more for the potential germs planted inside than the issue of tetanus.Cleaning the wound quickly with soap & water is the best initial action.Even a superficial scratch can allow the tetanus spores to set up in your body & cause tetanus. If you are > 5 years out from your last tetanus, or never completed the primary series, you should consider it.
Possibly. If your last tetanus booster was more than 5 years ago, it is not a bad idea. If it was more than 10 years ago - definitely. Keep up your immunity.