I have been working through the Ruby on Rails 2 Create a blog in 15 minutes and it only took me a few hours, but was much quicker and gave me a better understanding of rails than finishing the book on rails I was reading.
After this I had wanted to add tags like most blogs and photo site now have. So I am currently working through a act_as_taggable tutorial.
The main point I want to make is that at some point the require_gem command has been replaced with gem.
incorrect: require_gem 'acts_as_taggable'
Correct: gem 'acts_as_taggable'