Hello,
I would recommend that you start by exploring or use cases and our getting started tutorial.
There are also a few videos shared by the community.
In your case, if you want your users to be able to post contents, I would recommend:
- The Front Office Publishing add-on
- The Authentication Forms add-on
- The Notifications add-on
That would yet be a great starting point. Do not hesitate to check the product pages, our documentations, and to ask for any additional information before you buy.
Regards.