Skipper Course
After doing the free courses by NauticEd, I recommend taking the Skipper Rank course.
This course is a lot more advanced and will talk you through sailing a mid-sized sailboat (23 - 50ft). I think it's a great, comprehensive course.
It will go over all the core sailing theory: Weather and Sea Conditions, Anchoring and Mooring, Slip Departure and Return, Communications & Navigation, Coping with Emergencies, Sailing (of course), and many more.
It has over more than 20+ hours worth of material and will give you all the basic knowledge you need to start sailing in real life.
After the online courses, you can plan actual real-life sailing lessons that will build on your theoretical knowledge. The lessons are available around the world. I recommend getting at least two sailing lessons to get a feel for the boat and the basics of sailing. More on this later.