Welcome to Sitepoint!
It really depends on what you want in the end? Are you planning on developing a CMS for a commercial project or customer, or are you doing it to extend your learning.
If you are trying to do it for your learning, then concentrate on your database design and handling so you have a good understanding of how your users, posts, comments and any feeds are stored. Consider Foreign Keys for better management on updates and deletes.
Start out simple, just do simple posts first. Once you get this working try implementing comments and next implement users and finally feeds or APIs.
Most the things that you'll find online either are very simple or almost finished products which are hard to study. I think it would be easy for you to continue to build on what you have initially learned and create small steps towards having a simple CMS.