1. Login to the site as an admin and go to Admin > Config > Horizontal Tabs.
2. From here, you can see all of the tabs you currently have on the site, and also add more tabs. In the top right corner of the site you can see how many characters you have used.
Note: A max of 100 characters of text can be used across all tabs. If you have used 100 characters or more, you will not be able to add more tabs.
3. If you have used less than 100 characters, you will see a green Add Tab button on the left. Click this to add a new tab to the website.