Hi
Maybe the first thing to decide is what you enjoy doing most. Most photographers specialise in an area of photography that they have a particular love for eg. weddings, portraits, product, general commercial, travel, landscape, architectural etc. This is an opportunity to get into something where you combine your hobby with source of income - so you might as well enjoy what you're doing.
Then I'd research the market for your particular area of photography. Many creative people get carried away with the dream of how brilliant it will be to earn a living from photography without fully understanding what's going on in that niche nationally or locally. For example - the past 12 months wouldn't be a good time to set up a construction photography business! Research can be online (list all photographers in your target niche, review their websites and make notes of the way in which they talk abou their business and how they interact with their market. You're trying to glean how they acquire new customers and if possible their pricing structure and 'packages' that they offer.
Take a look at this London Freelance site and get a feel for the 'typical' pricing listed NUJ Freelance Fees Guide: Photography - Corporate work - rates
Decide upon a trading style - what you intend to call your business - needs to be credible and above all memorable.
Investigate various options for creating a website with your new business title, obvious sections are home, about, portfolio and contact us. Research 'the best' sites in your target photographic niche and make notes of what makes them work well - might be good, clear navigation and/or high impact images
Start to develop a view on how other established photographers in your target niche appear to market their businesses - website, Google Adwords
http://bit.ly/QJcrQ, local / national / niche press advertising, direct mail etc and construct an experimentation marketing plan where you try out a number of marketing activities and see which works best for your business. A lot of online promotion and marketing can be done at little or no expense - just effort on your part. You can find loads of web tools here
http://www.adamcoupe.com/resources.htm
Good luck
Adam
www.adamcoupe.com/portfolio