Before writing an OS, I would recommend writing some other programs first. Games are a great way to learn the Lua programming language and ComputerCraft specific APIs. I would begin with a simple game with entirely key-based controls, then move on to some requiring mouse input. As you want to create an OS based on monitors, handling monitor_touch events will be necessary - and they are almost identical to mouse_click.
After you have knowledge of Lua and most (if not all) of the ComputerCraft APIs, you should start on advanced topics, such as windows and coroutines. You'll also have to think hard about what features you want in your OS - if security is something you want, you should look into encryption and hashing, and when to apply each.
Once you have mastered the most advanced topics of Lua and ComputerCraft, you should make a final decision on the features of your OS and begin planning the separate files you will need. It is possible to create a single, huge file, but not recommended for a project as large as an OS. To give you an idea of the scale,
OneOS is over 120,000 lines of code. You might make that, you might not.
I feel many users rush to create their own "personal" OS because it sounds cool, and end up publishing nothing special. The fact that you want to focus on monitors is a good sign, as you already have something that will set your OS apart from others. I consider the vast majority (85% or higher) of the topics in the "Operating Systems" section of the forums to be unworthy of the name "OS". The single most important point of an OS is to add some sort of additional functionality - something you can't do in CraftOS.