Adding Categories to Pins in One Month Rails App

Adding Categories to Pins in One Month Rails App

I am really new to rails, and I am learning through one month rails. It's
an app with lots of pins in which users can upload images with. I would
like to add categories to each pin, so that when I look in to the specific
ex."bikes" category, all the bike pins will show up.
I have searched around and tried a number of ways with categories,
categorizations, belongs_to, has_many associations, but I still can't
understand how and where to get it started.
Specifically, I would like to set up a number of categories somewhere (not
open to any users to create new categories)which the pins belong to, so
that users can select a category (dropdown menu)when posting/edit a new
pins.
Your help is much appreciated. MW