One of the things I always recommend when creating a new product like an online course is to start with a MVP, a minimum viable product. This gives you the opportunity to get feedback from your ideal customers and make any necessary modifications prior to your product launch.
The best way to launch a minimum viable product for an online course is through a beta test.
However, before you launch your beta, be sure that you conduct your customer interviews about your future online course. You still need to validate your course idea prior to launching your beta. You also need a solid outline of your course content, so you can set up your beta course landing page.