Welcome to the My Online Karate community. You should start the modules in the correct order as this is what would happen at a club, if you try to skip too far forwards the system will ask you to go back and complete the modules.
Once you have completed a module you can mark it as complete and move on to the next module. It is very important to return to all the modules on a regular basis and practice them until you are confident you can do it proficiently. Some of the modules i.e putting on a karate suit, tying a belt etc will be quite easy, however, the actual karate moves you will never be able to perfect as they take a lifetime and will be part of your ongoing training. The more you practice them the stronger and better you will be.
Have fun, train hard and look forward to seeing your results in the future.