typically it depends on setup, whether or not you're authenticating against a Windows domain or against local files.
If you're going against local files, then you'll want to run
smbpasswd -a <someuser>
to add the user to the <path to samba>/private/smbpasswd file, once there, you give the password that you've created to the user and if they have interactive shell access to the machine, then going forward, they can periodically change their samba password by invoking smbpasswd
if this isn't clear enough, I'm sure with a little looking I can find the section in the faq on the samba.org site - jeff