Tsz attempt one no tests

I have already learned a ton here. I had no idea how to use Rspec outside of Rails. Having said that I am completely lost on when to write tests for this. I have the app working but this is my normal issue and why I joined Prime. I really need to get used to and learn how to write tests. If anyone has any tips for how I should approach this from a TDD mindset, I would appreciate it. Please don't just post the tests. I want to figure it out.