There are a couple of things going on here that should be tackled separately.
Creating a Category selector
You have the right idea. You will need:
Category Item template: this is the template that will be used to create category items
Category Item Picker (component): most likely a code-only component that will use Asset.GetFilterList to discover all Category Item content assets in a particular location, and then render an Input.ShowSelectList control.
Once you have the Category Item Picker component, add a new section to the Article template definition that uses a “null” layout and your picker component. The latter will have the category picker appear on a new tab that matches the section name.
Adding tabs to your input forms
There are two ways to do this depending on whether you are using Component Library or plain templates.
If you are using Component Library, then each tab in the input form corresponds to a “section” that you create when setting up a Template Definition. A section is a combination of a layout and the components to place in that layout – see Surety Press Release Template definition (id 8366).
If you are using plain templates, then you need a combination of:
NOTE: It is best to ask generic questions ("How do I create tabs in my input form using Component Library") on the Community site. If you have a problem that requires you to list specific asset ids, that would be better handled by contacting Support or an Account Manager.