The koans are broken out into areas by file, hashes are covered in about_hashes.rb, modules are introduced in about_modules.rb, etc. They are presented in order in the path_to_enlightenment.rb file.
Each koan builds up your knowledge of Ruby and builds upon itself. It will stop at the first place you need to correct.
Some koans simply need to have the correct answer substituted for an incorrect one. Some, however, require you to supply your own answer. If you see the method __ (a double underscore) listed, it is a hint to you to supply your own code in order to make it work correctly.