Bad Idea....
I know this is long but READ THIS!
I can tell you from experience that this is not the best idea.
I have alot of studio experience. I have a large home studio and I have done two apprenticeships in 2 different commercial studios. I have spent alot of time in recording studios and currently I work as a recording engineer in one. The only way to learn is HANDS ON. You need to be there in the studio, getting your hands on the gear in real time. You need to see what problems arise and figure out how to fix them. You can read all the material you want, and watch all the videos you want, but if you are not in the studio doing it, you will never fully understand it. You can't become a great mix engineer by reading about it. You need to do it over and over and develop your ear. It is an art and like anything else takes time and practice, Things like mic placement come from trial and error and experimenting with different mics, rooms, distance etc....
Here is what you should do....
Go to the library and get books on sound waves, recording, mixing, anything you can get your hands on. Then take the money you were going to use for the online thing and get yourself a couple mics, some studio monitors and either some computer software or a multitrack recorder. You will learn a ton more by reading these books and experimenting at home then you would with the online course. In the end you will need to go to a real studio and see if they will let you just hang out and watch. Maybe wrap some cables or something. You will have to do it for free but you will learn a ton from watching these guys and actually being there. This is how most guys get started. For real this is the only way to do it. If you have any questions I would be happy to answer them.
