Learn by doing. One way that I do is try to implement existing sites. This lets you concentrate on building the product instead of designing how it will look and feel. For example. Building this site in RoR would be a great exercise as the scope is large enough to include many features but not so large as to be impossible.

That said I wrote this blog entry a while ago that explains the main resources that i used to learn RoR. http://www.productcriticblog.com/2007/03/top-6-resources-to-learn-ruby-on-rails.html