It sounds like you want to access your Linux machine from outside your LAN, correct? This can be done with secure shell
(SSH for short), which as the name implies gives you access to a remote machine via a shell (command line). You can run graphical programs over SSH as well and this is known as tunnelling. On your Linux machine, you'll need to have an SSH server running so that you can connect to it. The default port for SSH is port 22. What your friend probably meant is to have your ADSL router forward any traffic to port 22 to port 22 on your Linux machine, so you can access it (read about port forwarding on Wikipedia
. It probably explains stuff better than I can!). Routers generally have a web interface, which you can use to change the settings (including port forwarding).