First of all log-in as an admin user. And then hover over the Forum Dashboard until the dropdown menu appears, then Forum and now click on New Forum. 

You will now be taken to the New Categories and Forums Screen.

Here you can add a Forum title and also give it a description.

If the forum should sit under an existing Category, then select a Parent Category via the drop down box. 

*If you do not choose a Parent, then this Forum will become a Category.

And choose the permission level of the Forum:

When happy, click on Publish

You will then be taken to the Forum & Category main page where you can re-order the categories and add a forum.

To add a forum to an existing category, locate the relevant category and then click on the + symbol in the blue menubar

This will then allow you to create a Forum title and description (as per above). The category will be automatically be added as the Parent Forum. 

Click Publish and the Forum will be live.

The Category & Forum will be live and available for members of the forum to add topics: