Unit Tests in Rails
Since you'll start your app by drawing up some models, you should start by writing some unit tests. When talking about Rails, unit tests test your models. Other models, such as integration and functional tests, will test your controllers and views.
All of your tests reside in the test directory, while the unit tests reside in the test/unit directory. Exploring this directory, you'll find it empty at first. This directory will either be manually populated by you or, more likely, populated by the scaffold generator. Since using generators to do the grunt work is preferred, generating your tests along with your models is a good idea.
If you go ahead and scaffold your first model, you can explore what it will create in the test/unit folder. In this example, I'll make a little microblog app. It won't have users, just posts. The posts are really simple, they just have the normal timestamps created_at and modified_at and a body. I'll start by running the scaffold generator and migrating the database.
$ script/generate scaffold post body:text
route map.resources :posts
$ rake db:create:all
$ rake db:migrate
== CreatePosts: migrating ====================================================
== CreatePosts: migrated (0.0198s) ===========================================