If you are a beginner, then I recommend to start with the course “Brush up your basic” (from Miscellaneous section). 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 “Brush up your basic” can be found useful to hone certain skills.
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.