How can I add Tags to my project and how can they be deleted?

To add tags to your project or delete them, you have to navigate to the project settings. Click on the wheel on the bottom left and go to Projects/Project Settings/Keywords or URL tags.

You can rename, move oder delete Tags with a click on the 3 dots next to the tags. In addition to that you can add Subtags (please find attached: structured Tagging)

New Tags can be added to the project by clicking the "+"-button.

You can easily organize your Keywords by drag&drop. 

Have a look at: How do I use tags?