Supposing you want to make bash your default shell, you'll use:
chsh -s /bin/bash
The chsh command allows you to change the user login shell. -s lets you specify the shell. Without -s chsh will enter interactive mode.
Sometimes, when you are busy or out and about and your iPhone starts ringing, you don't want to pick it up, but you know it might be so-and-so with that important call you've been waiting on.