A dietician is a trained expert on nutrition. Expertise can vary from sports nutrition, weight loss nutrition, geriatric nutrition, etc. A dietician is able to assess your current nutritional habits and help you create a healthier diet that is usually based on the U.S.D.A. Food Pyramid or other Government standards depending on where you reside, but can vary depending on the dietician's beliefs and level of experience.
Would it be worth your time to see one? Maybe. It depends on your current level of knowledge about nutrition and healthy eating. If your current diet consists of fast food and processed foods and you see no problem with that, you could use a dietician's guidance. A few sessions could point you in the right direction and then you could be off and running.
On the other hand, if you are relatively healthy, dine on whole, fresh foods, and exercise regularly, you could learn a lot by cruising some good websites. Good luck!