Do you have Nodes (Category) and Product types created in your solution.

For example, the Foundation example site has Fashio Node (Fashion Category)

https://github.com/episerver/Foundation/tree/develop/src/Foundation/Features/Category 

You will need the model (i.e. metaclass) for Catalog UI to create new node and entry. It does not have to be strongly typed type, but it is most easy that way.

I would suggest to go with Quicksilver instead of empty site, because you have the basics setup for you (you can also try Foundation but it is much more "full fledged" site which can be overwhelming)

Agree with @Quan, the Quicksilver sample code is the best  way to start Episerver Commerce learning. 

https://github.com/episerver/Quicksilver

Ok, I see the CategoryController and the FashionNode in Quicksilver. Thanks!

Are there some tutorials that can help me better understand how to work with NodeContent and ProductContent? 

Actually, I found this: https://world.episerver.com/documentation/developer-guides/commerce/getting-started/creating-a-start-page/ which contains instructions on categories. I was looking in https://world.episerver.com/documentation/developer-guides/commerce/getting-started/adding-e-commerce-functionality/ since I already knew how to create the start page, I didn't realize there was a lot more than the start page in that guide. They should really move it to the e-commerce-functionality page. Thanks all!