In order to create any content within a course, you need to have a course shell created first. If you are not a Canvas administrator at your school, you might want to talk with someone who is. Often, this person is in the Online Learning / eLearning / Distance Education team and would be able to tell you how empty course shells are created. Course shells are probably created either via the school SIS (Student Information System) or manually created.
Once you are enrolled as an instructor within a course shell, you can begin building your course content. The "Modules" screen is where you would build out the structure of your course...which includes things like Pages, Discussions, Quizzes, and Assignments. But, you can build all your content via those screens as well, and then you can return to the "Modules" screen to add all your modules and then add the content you built from those other areas of your course into the "Modules" page. Or, you can begin from scratch on your "Modules" page to create your Pages, Discussions, Quizzes, and Assignments. Either way works...it's just a matter of preference.
I might suggest watching some video tutorials if you are looking for ideas on how to get started building your course content. Here are a couple links I typically recommend for people:
- Set Up Your Canvas Course in 30 minutes or Less - Instructure Community (canvaslms.com)
- Canvas LMS Tutorials for Online Teaching - YouTube
The second link I've provided is a set of tutorials by Ashlee Espinosa. I've not watched all of them, but they are very good, and I know lots of people recommend watching here videos.
As for Canvas Commons...Commons is a place to share course resources with others at your school or with the wider Canvas community. You'd have to have content built in your Canvas course in order to share anything to Commons. Or, if you had an empty course (or even a course with some content in it), you could search the Commons repository to import content into your own course.
I hope I am interpreting your question correctly. Hopefully I've given you some ideas and things to look at here. Please let me know if you have any questions or if I've totally misinterpreted what you were asking. Thanks!