Had I a nickel for every one of these threads....
Putting aside the debate about how long you've been shooting, the first question is: Do you have the skill and equipment to work as a professional? Are you confident that you can walk in to any situation, even (or especially) one which is completely different than that described by the client and pull off a quality job? If your answer is anything but an resounding 'Yes', then spend a little more time learning.
As has been said, the business of photography is very little about photography and very much about business. Taxes, insurance, taxes, marketing, taxes... If you're not at least reasonably well versed in entrepreneurship, than I would strongly suggest you contact your nearest adult-ed facility and take every course you can relating to the running of a small business.
Assuming that you do have the requisite skills, you need to actually get a business established. Between insurance, legal consultations, filing fees, license fees, etc, I would expect to shell out somewhere between three and five thousand dollars. Now... once that's done, and you actually have a legitimate business, you can start marketing. . Marketing is a hugely important part of your business, and will (or should) take up the bulk of your business (as opposed to your behind the camera) time.
How you approach marketing depends on many factors. First is your target market, you need to drive your marketing to the people you want as customers. To do that, you need to know who your target market is. For instance, if they're 16-25 year old in a major urban centre, then facebook and other social media is a good start. If you're like me and your target market is 55+ and commercial, then more traditional (newspaper, etc) may work best.
However you go, it's a LOT of work. It can be very rewarding, but it is NOT something to be undertaken lightly.