How to add tags in craft CMS 3
Here are some basics:

First, tags and categories are two different things. As a first step, think long and hard about how you want to organize your content. Keep in mind that Categories can be hierarchical, whereas Tags are not.

To start using Categories, you must first create a Category set, say, "Blog", then specify categories within that set, e.g. Beauty, Health, etc. Note that you can assign fields to a category set, so you can associate things like icons or brief description for each one! 

To use your Categories in entries, first create a Category field(s) and then add it to one or more of your channel entry types. then, when you create a new entry, you can assign a category to it.

To access entries by category in your templates, refer to this section of Craft Docs