Hi. I'm a 16 year old inactive girl; 85 pounds and 5'2. how many calories should I eat a day?

Evolution. Don't worry about calories. Eat 2-4 meals and 1+ snacks each day composed of fresh produce, whole grains, nuts, seeds, beans, nontoxic seafood, lean beef/poultry, and whole fat dairy. Such a diet should provide sufficient macronutrients (protein, fat, carbs) and micronutrients (vit/minerals) to meet your daily needs. Although you are 'inactive' you need to find some daily activity you enjoy.
Get active! About 1600 calories per day but adjust according to your activity level.