To earn or make a living from photography, you need to be able to master two very diametric skills. They are firstly Photography and then business, in that order.
Mastering the basics of photography are the prerequisites of becoming a commercial photography. Having a good eye for photography will set you apart from the crowd. Despite all these, you still need to know how to run a business. This can be very hard for a lot of people as most are only inclined towards either photography or business, very rarely both.
The most successful commercial photographers can handle both areas well or they are smart enough to find a partner to take care of the business side. Running a business is very different from taking good photos. The skill sets and requirements are very different.
It is very important to do an honest self appraisal of oneself before taking the plunge into commercial photogrpahy. You need to do a SWOT analysis and take the appropiate actions. SWOT = Strengths, Weaknesses, Opportunities and Threats.
The above are from materials I read from books, articles and my personal experiences.