There are many things you can do to secure samba from passwords to kerberos.
First, to access the share, you will be using a user name and password configured on the samba server.
I assume you already have a (login) user.
You then assign a samba password to the user
sudo smbpasswd -a username
sudo smbpasswd -e username
You then define your shares in
sudo -e /etc/samba/smb.conf
gksu gedit /etc/samba/smb.conf
In the section where you define your shared directory
comment = private share
path = /path/to/share/point
browseable = no
read only = no
guest ok = no
After making those edits, restart the samba server.
sudo service samba restart
There are a number of security features for samba, see also
Ubuntu Server Guide Securing Samba
Ubuntu Community Wiki Samba Server Guide