I weight 120 lbs and want to maintain my weight, how many calories should I have a day?
To know how many calories you should have a day so as to maintain your weight, you need to use a calorie calculator. You do this by filling in the fields required and then submitting. Alternatively, you can contact a dietician for more advice. A website where you can calculate the number of calories that you need is http://www.realage.com/food/personalized-calorie-count or http://www.freedieting.com/tools/calorie_calculator.htm.
As many as you burn. If you're very active you should eat more calories to maintain your weight than if you're relatively inactive.