How to change the Shell in Mac OS X Terminal

To permanently change your default Terminal shell:

1. Choose Preferences from the Terminal menu.
2. Select the option "Execute this command (specify complete path):"
3. Change the selected text entry from /bin/tcsh to reflect a different Terminal shell, such as:
   /bin/bash
   /bin/csh
   /bin/zsh
   /bin/zsh-4.0.4 (Mac OS X 10.2.8 or earlier)
   /bin/zsh-4.1.1 (Mac OS X 10.3 or later)
   /bin/ksh (Mac OS X 10.4 or later)
4. Close the Terminal Preferences window.
5. Quit and open Terminal again. The first and subsequent new Terminal windows open with the shell that you designated in Preferences.

http://support.apple.com/kb/ta27005