Here’s the minimal steps required to install Jekyll on a Debian system so you can test things out before pushing up to github pages (or wherever).

$ sudo aptitude install ruby ruby-dev rubygems nodejs
$ sudo gem install jekyll

As far as I know there aren’t any other required dependencies that won’t get pulled in automatically.

I know there are a ton of instructions on installing Jekyll out there, but nearly all of them miss ruby-dev and nodejs which are required. Without you’ll get some subtle and vague errors, especially if ruby-dev is missing which will cause the gem command to fail.