At the start of the small projects I like to write out a description of the system in plain English. Then I identify the nouns. That set of nouns gives me an idea of the tables in the database. You can also identify the verbs and match those up to methods in your code, but I find that less useful.
An author logs in to create posts. Readers read posts. Readers log in to create comments about posts. The author can modify and delete posts. The author can group posts into categories.
Nouns: Author, posts, readers, comments, categories
I am going to call authors and readers "users." But the other nouns will go straight into the database. So my tables are going to be
users
posts
comments
categories
Now that I have my database prepared, I will use the bake script to generate my models and controllers.