When you drop to the terminal
via Applications -> Terminal or Ctrl+Alt+F1 etc. bash is used by default to interpret your commands. If you wish to use another command interpreter (like zsh or fish), how do you get this to start as default?
Change your default shell. There is a command-line way to do this:
$ sudo usermod -s /path/to/newshell username
but it might be easier to do it from the GUI settings.
System -> Administration -> Users and Groups -> [select user] -> Advanced Settings -> Advanced
then choose the shell from the drop down list.
If the shell you want isn't in that list then it's probably not installed.
You'll need to log out and back in again for this to take effect. You can check which shell you are running in a new terminal by running "ps".
If you're very curious, the default shell is stored in /etc/passwd (which doesn't really have passwords despite the name).