My advice when starting is establish your goals - collector, reader, investor, etc. People on the business/investinf side tend to focus on comic book key issues - first appearances, deaths, iconic covers, first of the age, famous storylines, etc
This is a great video on a blue-chip Grail book that every collector should have on their radar. Kind of like with stocks when people say as Apple goes the market goes, when it comes to Comics as giant size X-Men goes, the comic book Market goes.
