The first step is to step away from the computer and start planning things on paper. If you just go diving into coding your objects straight away, it will most likely end up as a huge mess!

Personally, I like to brainstorm the idea as the very first step. Start with whatever it is you're aiming to achieve ("Build a CMS") and branch out from there. Only when you know what exactly it is that you want to build, can you start building it.

Take your time and try to think more widely than just creating a list of 'systems' that you'll need.