The course is separated into modules. Each module will have a specific topic in mind. The lessons are designed to go in order and you can click through them as you move. Many times there will be extra problems you can try. These are meant to have you mimic what was going on in notes.
If you have not used R before you should follow the first 3 modules in order:
Then you can pick any of the other modules as needed for your work or pleasure.