Contrary to Java, that requires a solid grounding before you can start using it, Ruby is pretty easy to approach. As long as you understand OOP (and you're willing to understand some functional-style programming), you can begin with Ruby by just goofing around and reading introductory tutorials. If you want to be systematic, the "pickaxe" book is still one of my favorites (http://pragprog.com/titles/ruby3/programming-ruby-1-9). You can also find an outdated HTML version of it on the web for free, if you want to check it out first (http://ruby-doc.org/docs/ProgrammingRuby/).
Paolo Perrotta, Europe
Author of Metaprogramming Ruby
I'd say that one requirement is to forget some of the things Java taught you (us). You'll need the willingness to approach OO from a different angle, because Ruby feels decidely "not" Java at times. If you don't allow yourself to "let go of Java" for a time, you'll end up writing Java code with Ruby syntax... a travesty if there ever was one!
Why's Poignant Guide to Ruby was the first real look I had at the language, a great (if not a little strange) introduction. Glad that it's still available.