Graphic Design is a sophisticated discipline that has developed over centuries, and the best way to learn it thoroughly is through a course. If you want to learn it for free, then you'll need to scratch around and see what you can find (Lynda.com is not free). There are lots of good books on various aspects of the subject, such as typography, so perhaps head off to your local library and see if that has anything useful. There are websites that provide tips on various aspects, too.
Perhaps read up on all the aspects of graphic design, make a list of all the important skills and find resources on each from wherever you can, such as typography. Good luck.