if you want to do that in netinfo, set it to '*' or just remove the property. Then the user will have no password, and can login and change it.
Here is what I recomend:
Login as your admin user. This is the user you created when you first installed osx on the computer.
Open a terminal, type:
sudo passwd username
where username is the username you wish to change passwords on.
Next, enter your password. Then enter the new password for the user you specified.
You can also do this in the login pane of the System Preferences if you're signed in as an admin user.