I’m only a novice myself, really. Rebol and descendants tend to have little documentation, but poking around the websites [0] [1] should be enough to get started.

[0] http://www.rebol.com/docs.html

[1] https://www.red-lang.org/p/documentation.html

(NB. Red is a descendant of Rebol; they’re not the same, especially with more advanced features, but they’re still similar enough that the documentation transfers over easily.)