A course format is a relatively complex type of plugin to create.
My advice for getting started would be to create a copy of one of the core plugins (e.g. topics), then create a copy of it and call it something like "mytopics" or "topics2". You'll need to change all the references to "topics" and "format_topics" in your copy to refer to your plugin's name instead.
Once you've done this, you should be able to switch a course to use your format, and it will look exactly like the core format.
Now, choose one thing you would like to change about the course format. Work out where in the code the thing you'd like to change is created, then make changes to that piece of code until it does what you want.
If you get stuck, ask questions here being as specific as possible about what you are trying to achieve, how you have tried to achieve it (the code you've changed and how), and what's happening instead (including the text of any error messages you see).
Once you've made changes successfully several times, you'll have a better idea how a course format works. At this stage, you may decide that you want to start over on a new format plugin, or continue making further modifications until you end up with the format you want.