Navigate to the section where you want to add the forum. Click "Add an activity or resource".

Provide a forum name and optionally write details/instructions in the "Description" field.

Select the "Forum type" dropdown and select from one of the 5 forum types.

A single simple discussion
Each person posts one discussion
Q and A forum
Standard forum displayed in a blog-like format
Standard forum for general use

Click "Save and return to course" and ensure that the new forum is present in the Moodle course main page.

Add a forum type