The usual number of grams of carbs in a serving is 15 grams. That doesn't mean that every serving is the same size. Fifteen grams of carbohydrates could be a cup of milk, a slice of white bread, two pancakes or half a banana.
There are a lot of resources available to help you figure out the serving sizes and gram counts. In the U.S., food labels are required to have nutrition facts on them. Carbohydrates per serving are listed on these labels, as well as fat grams and other information. If the food you are going to eat doesn't have a label, there are books and pamphlets with carb values for many different foods. Websites such as Calorie Count, also can tell you the carbs per portion for hundreds of foods, including restaurant and fast food items.
"Using The Diabetes Food Pyramid." Making Healthy Food Choices. American Diabetes Association. 29 Nov 2006