The OLCP Wiki has got Forth Lessons for everyone to enjoy. Forth is a pretty neat language!
You may have noticed that although access to the Forth shell is explained on this page, it doesn’t work. The reason can be two-fold. First, you can only access the Forth shell if the firmware security is disabled. Second, on newer machines you access the Forth shell by hitting the escape key at boot time.
This page explains how to gain access to the Forth shell on machines that have got the firmware security enabled (G1G1 owners, that means you). You need a developer key to unlock your firmware, the request takes less than 24 hours to be fulfilled. Please read the page closely (disable-security twice!) and heed their advice of disabling firmware security (you can always enable it later).