A regular teacher can't delete a course. Administrators and managers (i.e. users with a role for which the capability moodle/course:delete is allowed) can deleted courses. A Course creator can delete courses they have created themselves.
To delete a course (as an administrator or manager):
Go to Settings > Site Administration > Courses > Add/edit courses
Click the course's category (to drill down the category tree) and find the course you wish to delete. Or use the "Search course" textbox at the bottom of the category list.
Click the "X" icon to the right of the course's name. to delete it.
You can delete multiple courses by:
Creating a new (temporary) category. You can name it "To be deleted".
Select and move the "About to be deleted" courses to that category ("To be deleted").
Delete the category ("To be deleted") and choose "Delete ALL - cannot be undone".