Guidelines vary depending on the strictness of your doctor. In general, an a1c 5.7 - 6.4% is considered "pre-diabetic". But many feel that this is sort of like being a "little bit pregnant". Basically, you have a dysfunction in your glucose metabolism that is not yet too severe.
Diet and exercise are the two most important things to moderate your glucose levels. There is no one-size-fits-all diet and you will have to check your blood sugars to determine what food are best for you.