To create a Forum, first go to 'Community.' The Forums tab will be open by default. Then, click 'New Forum.'
Choose if you want your Forum to be 'Public' or 'Private.' Give your Forum a title, and a brief description. If your Forum is Private, you can then 'Add Members' by clicking on the designated button and selecting the users you'd like to add.
When you're done, select 'Done.'