The hardest but most important part on your way to becoming a casting director is getting that first job as an assistant or an intern. This puts your foot in the door and allows for promotions. Because this career is not taught at universities, an internship is the ONLY way to learn the rules of the road and the importance of your future job. Some important skills you will need for this initial internship is a willingness to work long hours, a passion for characters and actors, and you must be willing to meet and get to know many, many people. This ranges from actors who are auditioning, the movie production team, the directors, etc.
Good people skills will take you very far in this career path. It is also a great idea to take a few acting classes because the more you understand what kind of work goes into acting and the different types of acting (method acting and the like) the better casting director you will become. If you do get this prized internship, be prepared for work that seemingly has little to no association with becoming a casting director at all. You could be sorting mail, retrieving coffee, being a personal assistant, etc. But this is teaching you how to network and be comfortable in a production environment.
The most IMPORTANT part of this internship is the learning process. Therefore, you must be open to everything. Ask questions of everyone you meet. This could be actors, other casting directors, directors, production teams, lighting crews, etc. Everyone you meet will have some kind of valuable information that you can take in and learn from. The more you learn, the better you will do and the faster you will advance.
The next step after your internship is becoming an associate casting director. This means you will work on your own small projects and have much, much more independence and many more responsibilities than you did as an intern. Since you now have a much more active role in the actual casting process, you will learn even more quickly. You will begin to learn the importance of meshing business goals with creative goals and how to achieve the perfect balance between the two. Too much on the business end and the film/tv show/play you are casting for will fall flat or be mind-numbingly boring. However, too creative and you lose a huge audience because most demographics are not into bizarrely creative works.
Taking on this career path is a huge decision, but if you are sure you can handle the long hours as well as the long ladder you must climb in order to become a casting director, then the payoff will ultimately be more than worth it.