The twelve rules discussed here are a useful starting point. They demonstrate what work with accessibility means, and they deal with relevant problems and solutions. They are also useful in continued practice, should you decide to stick to such basic issues for the time being. It is not a bad idea to do so for some months before starting to study advanced accessibility. That way, you will have some routine with the basics, and you will probably have noticed some accessibility issues that are not covered by the rules.
Among other things, the rules do not discuss the following topics:
As you probably guess from this rather incomplete list, there is much more to be learned in accessibility. After learning more about accessibility, you might write your own additions or extensions to the twelve rules, or a different set of basic rules for yourself or your organization. On many sites, some additional rules might well be more important than the twelve rules.