Ways to learn Factor
There are many great resources for learning Factor.
Documentation is an important part of every good Factor vocabulary. Consequently, Factor's amazing and extensive docs are available offline, searchable right from the GUI Listener, or on the command line. These same docs, equally searchable, can be found online.
Factor's GitHub repository hosts a Wiki.
Björn Lindqvist, one of Factor's developers, has a repo full of interesting tips and tricks. It's not updated very often and is WIP (and much of its content is being moved to the GitHub Wiki), but it's still very cool stuff.
Learn X in Y Minutes is a good resource for many languages, and Factor is no exception. Its tutorial is not extensive by any means, but it is a good reference and enough to get you started.
Rosetta Code is a programming chrestomathy site. On it, among the hundreds of other languages, is Rosetta Code:Factor, where you can find solutions to many problems, and compare with languages you know. It also serves as a good reference and starting point for solutions.
Lastly and maybe leastly,
Stack Overflow. There's not a large community (okay, about three users including yours truly) but ask there if you're really, truly stuck and someone will surely help you out.
The Mailing List. Here, you can ask about anything Factor-related and a collaborator will answer helpfully.
Factor is on IRC! Come join us; we're happy to help.
Learn and read about the Common Lisp Object System. The CLOS is widely regarded as the most advanced and innovative object model in the world, and Factor's object model is heavily based on and inspired by it.
In a lot of ways, Factor is a Lisp. It's a postfix, point-free, functional, inside-out-and-backwards funny-looking one, but many ideologies are the same. Because of this, it may be beneficial to learn a Lisp, preferably a Lisp-1 in which functions and variables share a namespace (because functions are values in functional programming). Yours truly humbly recommends Scheme, or, for something more modern and usable, Racket, a descendant of Scheme.
Finally, Factor is written almost entirely in Factor. So, read the source code, and search the docs for words you don't know.