About 300 calories per day are needed above your normal intake during the second and third trimesters of pregnancy (500 additional calories a day if you are under 18). As examples,you can easily add 300 calories by eating a piece of bread, an ounce of cheddar cheese, and drinking an eight oz. glass of low-fat milk.
Ask your doctor for the recommended weight gain for you during pregnancy, and follow a plan for eating nutritious foods to gain that healthy weight during your pregnancy. Pregnancy is not a time to lose weight or to "diet" for that purpose.