I want to follow just one course. Google is giving me a lot. Something like codecademy.com which is perfect for programming for beginners. What about for graphic design using photoshop?
My personal view is that Graphic Design is not really something you can learn without critique and peer review, I would suggest enrolling in a course if you are interested in design principles and how you can apply these.
However when trying to learn a new Adobe program, I find Adobe Classroom in a Book useful. I'm currently using these to learn After Effects and used them to learn InDesign before I went to University.
Also check out these previous questions How do I learn to be creative? & What is the single most influential book every designer should read?
But again, knowing some principles is OK, but you only really get better with peer review (and this never stops, no matter how long you have been in the industry).