That said, this is not as tough as you may think provided you really "have the goods" and are willing to invest in the tools you need to close those sales! No matter what you do - if you are, in fact, well-positioned to provide the service you say you can offer - concentrate on the benefits of what you can do for your prospects. Give them meaningful info about your deliverables and your expertise (you're creative, you have quick turn-around times, you are competitively priced, etc.) within an organized sales system, using professionally designed, written and printed materials.
If you want people to trust you, you'll also need to develop a distinct visual identity, using concise and compelling language to describe what you'll deliver to them, and why your firm is a great option. If you need help developing a strategy and the right sales pitch, reach out to a marketing mentor. Shop around and get other business owners' recommendations so you can get the rest of the team you'll need. These include a graphic designer, a marketing copy writer and a web master. These folks can help put together your company's professional, unique 'Face," which in turn will help you sell yourself and your services!
When describing your competencies, choose specific skills that are highly valued to your particular prospects (which means you have to truly understand your target market(s)) and publicize these skills in your collaterals - website, intro e/mail, brochure, sales letter, or through cold calling. Are you very organized? Do you possess a level of expertise though a degree or special training ? Even if your experience is from another position at another firm, it's perfectly acceptable to use your past to describe what you bring to the table in your own firm. Bottom line is that most companies do this on a daily basis - they build new relationships and make new sales by leveraging past successes.
With the right tools, a grasp of your clients' needs, a professional "face" and a smart sales system, you should be closing deals in no time!
Wishing you success!