Categories - A road map for your customers
So the first thing you need to do is find your categories which will be on your WordPress dashboard under the posts title:
Click on categories and you will see which ones you have created or if you haven't created any you can create them here as well as on your post.
Now you can type in your category and it will be added to all your other categories. These categories here are on my practice site. It is always good to practice on a site you don't mind messing up. Once you have learned the technique then you can apply it to your site that matters.