There are two ways to create a tag. The first and most intuitive way to to attach a tag from the header of the note details view. Click on the + sign next to the tag icon and type in the tag name. If the tag is not present, then a new tag will be created and attached to the note.
The second way is to use the tag list view. Open the tag list view by clicking on the tag icon. Click on the + sign next to the Tags label and enter the tag name.