Help with basic category setup

Hey all, I have created a small app that collects links submitted by
users. I have now decided that I want to categorize those links into
several different categories.

Being very new to Rails, I am unsure as to how to go about doing this.
I want to create a predefined list of categories, and when the user
submits a new link, they choose the category from a dropdown list.

I assume that I will need a Category model, but how do I define that
predetermined list of categories in the model? Can anyone point me in
the right direction as to the best way to do this? Maybe some sample
code snippets? Thanks!