Best Practices Add a category to a post
How to assign a Category to the Post.
Open the selected post and on the right hand sidebar, scroll down to categories and click the desired category. Update your post.
Alternatively you can add a category to a post by going to All Posts on the left hand sidebar. Hover over the title. In the Quick Link Options, click Quick Edit 
In quick edit select your category. Again there are more choices. You can add more tags (remember the comma after the tag) or make this post sticky(this means make them appear above the other posts) or you can change the status of the post(not category) to draft or Pending Review.