How do you change the default shell for ALL USERS to bash?

adduser

The adduser defaults file is /etc/adduser.conf. The default shell defined by the DSHELL variable is /bin/bash by default.

useradd

Most likely you don't need this because useradd is a very low-level utility, and it's hardly ever used directly.

If you use useradd, edit the /etc/default/useradd skeleton file (don't forget to make a backup though).

Set the SHELL variable to /bin/bash instead of /bin/sh.

Now every time you use useradd to add a new user bash is automatically their default shell.

Already existing users

If you want to change the shell of already existing users you have to edit the /etc/passwd file (please make sure to back have a backup of it).

Here is a description of the columns

1. login name
2. optional encrypted password
3. numerical user ID
4. numerical group ID
5. user name or comment field
6. user home directory
7. optional user command interpreter

In that order separated by colons (:) like this.

root:x:0:0:root:/root:/bin/bash

For more information about that file consult the man page man 5 passwd.

As Octavian pointed out, the way to change the defaults depends on the way you're creating the user. I tried creating a new user through my Gnome Settings just now, and it seems to follow /etc/default/useradd, so that might be your best bet. For existing users, the safest way to change someone else's login shell is with usermod:

usermod -s /bin/bash $USERNAME

If you're not root, you'll need to sudo that. An alternative is to sudo into the user you want to modify and just run chsh, like this:

sudo -u $USERNAME chsh -s /bin/bash

It's best to avoid editing /etc/passwd by hand, because a mistake in there could break all sorts of things.

If you actually want all users on the server to have bash (which was the question actually asked), you can run the command:

sudo dpkg-reconfigure dash

And then choose NOT dash. As explained in this answer,

How Can I Make /bin/sh point to /bin/bash?

Not only does that set bash as the default shell, but repoints sh and the man pages correctly.

Hope this helps.