I think if you know nothing about nutrition, you have specific health issues that diet helps to manage and you feel you need more support in the form of a prescribed diet, a dietitian can help. Understand that registered dietitians must adhere to specific guidelines with regards to diet, mostly a low fat diet that usually means sticking to the latest food pyramid or plate diagram.
All that sort of information is easily available online for free. Check out http://www.eatright.org/
. But if you need and actual person to talk things over and help you stay accountable, by all means, go for it.
Dietitians are required stick to those guidelines only. It is why they are licensed, and are considered a part of mainstream medical practices, like doctors. A nutritionist not licensed but may follow similar guidelines. They, however, are more free to make their recommendations based on alternative medicine.