Familiar shoes I've walked in. Build something more than the typical blog or book inventory site you see referenced in tutorials. If you can't think of something to build, then think of one of your corporate Java apps and build that in Ruby/Rails from scratch. Or if you want a side project then pick something and literally copy it in your own way. Like a photo sharing site or whatever.
Things to include in your example app so you can learn the ins/outs of how rails does it:
- Nested models: comments on photos and how to organize the models, many to many relationships
- Rendering Partials: you can factor out pages into partials and essentially load them anywhere in the app
- Gems ecosystem: check out rubytoolbox (.com I think) for categories of gems. These are like Java JAR files you can include via Maven. The popularity and activity level are good ways to assess usefulness (outside of requirements)
- Host your own server: learn how Rails apps run on a server and different options for running it. There are providers out there like Heroku where you can just deploy to them but I think it is best to understand how these work. You probably understand how Tomcat or other Java containers work, where to look for log files, how it might interoperate with Apache HTTPD, etc.
