If you are a beginner, then I recommend to start with the course “For beginners” (coming soon). After studying it you can move on to the other courses. In every course the classes start with the simplest ones and gradually progress.
If you are an intermediate or advanced level, then you can start with any course depending on what you find interesting or relevant to practice. Even the course “For beginners” can be found useful to brush up certain topics.
If you are a teacher, then you might find all the courses useful as you can use the material there to give you many inspiring ideas and exercises for your local classes.