Typically, forum threads are created by website users, but admins can also start discussion threads to steer conversation.
Go to Content > Community > Forum.
Click Add a Thread.
Set Publish or Archive dates, as appropriate.
Update the Comments settings, if needed. These will be pre-populated based on the global forum settings in Content > Community > Settings > Forum.
Add categories, as appropriate. Categories are not necessary in most cases, but they can be managed in Content > Community > Settings > Categories.
Restrict the thread, as needed. The Forum page can be restricted, so thread-specific restrictions are only needed if the discussion is available to a limited group or if it will be included on an unrestricted page.
Click Next.
Add content to begin the discussion. The Title and Content fields are required. The Icon field is optional. The Excerpt field is only needed if a Forum List content section is being used on another page of the website.
Click Next.
If a future publish date wasn't set, click the Publish button, when the discussion should go live on the website.