Mathieu's forgotten to give you some extra information.
The file you need to add the [myshare] area to is usually /ect/samba/smb.conf
You should change valid users = mary fred to
valid users = (whatever usernames you want to use)
When you're done, you need to save the file.
Then, you must open a command prompt (or console) and type in
smbpasswd -a username
Substituting "username" with whatever username you included in your smb.conf file.
It'll ask for a password; give it whatever you want to use as a password. Hit enter again and you're set, you have SAMBA configured and ready to run. You will have to restart the SAMBA service, so that it will read the new configuration.