Before setting the shell, you should probably make sure whatever program you want will work as a shell. "Valid" shells are pre-configured in /etc/shells; if what you want to use as a shell isn't in that list, I would consider carefully whether you want to use it as a shell or not. You could try it with a test user first, to make sure you can login okay using it as a shell.
But on to how to change your default shell.
For individual users, you can change their login shell using the chsh command.
chsh will accept the full pathname of any executable file on the
system. However, it will issue a warning if the shell is not listed in
the /etc/shells file. On the other hand, it can also be configured
such that it will only accept shells listed in this file, unless you
are root.
To change the default shell for all users, it will depend on how you add users. For example, if you use the adduser command, you can configure the default shell in /etc/adduser.conf:
$ grep -i dshell /etc/adduser.conf
# The DSHELL variable specifies the default login shell on your
DSHELL=/bin/bash