Well if you are just getting started and wanting to learn the basics I suggest you get your hands on a textbook before spending money on a course.
I started with "Refrigeration and Air Conditioning" by J.W. Jones and W.F. Stoecker 2nd edition (1982). It may be old but as far as the basics are concerned it doesn't really make a difference. It is out of print but you can find it in various places online for cheap or maybe even for free as a pdf. I only recommend this one because it is what I used and though I thought it was good I am sure there are others that would work just as well.
If you have any problems understanding the material in one of these textbooks you'll have to back up to basic thermodynamics, fluid mechanics, and heat transfer textbooks... It all depends on what you know now.
I would suggest starting like that and using online forums when you have problems.