First of all becoming a working professional photographer has to do with one main element; Becoming recognized in the field. People don't only buy your art; they also buy your credentials. In a sense, their buying what you represent. Often those things you represent will be of intrinsic value to your customers. Such as with whom you've worked in the past, where you've been published, how long have you been a photographer, where have you traveled to do work and so on?
Secondly you will also want to get as much education under your belt as possible. Throughout your education as a photographer you should also be taking as many photographs as possible to help build your portfolio. You will need to ensure your photography portfolio stands above the rest by paying extra special attending to detail. Look at the book itself, the paper it's printed on, the font of descriptions, your borders, your themes, picture sizes, colors, layout and so on. Do not simply fill a scrap book or photo album with photographs. Once finished, you should be able to walk away from your portfolio and be impressed with your own work, which is not easy task for many photographers, as most of them are their own worst critics.
Further you should also start looking around for assistant or internship work. Often this work doesn't pay well but still it will help you learn more about the industry and art. You should try and work under the most respected photographers you can find. By doing this you are adding to your credentials without knowing it. You only want to surround yourself with the best and present your best side all the times.
Moreover you will also need to have a strong sense of business acumen. If you are planning on being like the 50% of other photographers who are self employed, you'll need to make sure you have a good sense of networking skills, pricing, public relations knowledge and on top of that have a strong sense of creativity and imagination. Being an artist and a business person will be no small feat. It will have you on your toes at all times.
Lastly, you'll want to narrow your professional interest to one or two main fields. For example you may want to work with press and other media, or you may be interested in photojournalism. Others may be interested in scientific or sports photography. Whatever your interest is, make sure you find the best people in that particular industry and study them. Model your success after theirs.
Remember, nothing worth while comes easy. There will be times when you think "what am I doing?". Stick with it and there will be a payoff at the end of the tunnel. There will be a ray of light in the end of the day. Most importantly, enjoy your whole journey. The success at the end wouldn't be as welcome if there wasn't the struggle at the beginning.