Rule out alternatives
Before you open SAMBA access to the rest of the world, have you thought about using a more secure service that's designed for wide-area network usage? If I'm not mistaken, SAMBA is designed for local/secure-ish networks, whereas alternatives, such as sshfs, scp, sftp, may be a better solution for you.
Ensure SAMBA lockdown
Before allowing SAMBA to be accessed remotely (over the Internet), make sure you set it up in such a way that you can control access to your files. How-To
In order for remote computers (those outside of your LAN) to access your SAMBA machine(s), you will need to set up port forwarding such that the TCP ports 135, 139, 445 as well as the UDP ports 135-139 are forwarded to your SAMBA machine. How will you allow access to multiple machines? I don't know.
Again, please look at alternatives to this solution, as I don't believe anybody would recommend using SAMBA over WAN.