Get started with herkforth
You should be able to follow the herkforth tutorials even if you don't know Forth.
If you know Forth already and want the minimum information to get started with herkforth you'll probably like the herkforth crash course.
Here's some documentation links that might be where you want to head next:
How to use the herkforth interface
And finally you may want to know some herkforth words