The following is a quick guide to reading the Nutrition Facts Panel. Start with the Serving Size Look here for both the serving size (the amount for one serving) and the number of servings in the package. Compare your portion size (the amount you actually eat) to the serving size listed on the panel.

Let the Percent Daily Values Be Your Guide

Use percent Daily Values (DV) to help evaluate how a particular food fits into your daily meal plan:

• Daily Values are average levels of nutrients for a person eating 2,000 calories a day. A food item with a 5 percent DV of fat provides 5 percent of the total fat that a person consuming 2,000 calories a day should eat.
• Percent DV are for the entire day, not just one meal or snack
• You may need more or less than 2,000 calories per day. For some nutrients you may need more or less than 100 percent DV.

More here: http://www.eatright.org/Public/content.aspx?id=10935