To grow your own CMS you need to be proficient in HTML/CSS, PHP and SQL, and it's not just about the programming, it's the layout and design as well.
If you were to create a Bulletin Board / Forum type system, you probably don't have to worry a lot about features/plugins and templates, but for a system to become popular, it needs to cater for the needs of those who use it.
That's why I would recommend using either Bulletin Board software, or Wordpress, Joomla or something similar, because these have been designed with all of this in mind.
If you create your own system, you will need to ideally do some detailed design to know what you want to add into it, or do this as you go along, which results in lots of program (script) changes, and a lot of messy code.
It's nice to do as a project, but if you consider the thousands of hours of development time that have gone into Wordpress, you would be better off building plugins/themes for Wordpress that made it do what you want.