You can use the controls in the Startup pane of Terminal preferences to set up the default window settings and type of shell used when a new window is opened in Terminal.
On startup, open:
This section contains the settings for the startup window.
New window with settings
To open a new window configured to a set of saved settings, select “new window with settings,” and then select the window settings from the pop-up menu.
Window group
To open a saved group of windows, select “window group,” and then choose the group from the pop-up menu.
Shells open with:
This section contains the settings for the shell which is used when a new window opens.
Default login shell (/usr/bin/login)
To open Terminal with the default login shell, select “default login shell (/usr/bin/login).”
Command (complete path)
To open Terminal with a different shell, choose “command (complete path)” and then you can enter the path to the shell.