I'm a 5'7" 158 pound, 21 year old female. I'm looking to lose 10 pounds with healthy eating & exercise. How many calories should I take in a day?

500/day less. There are about 3500 calories in one pound, so to lose 10 pounds, you will need to burn about 35, 000 more calories than you take in (through eating fewer calories each day and burning more through exercise). If you did no exercise and ate 500 cal per day less than usual you would lose about 1 pound per week/10 lbs in 10 weeks. Daily exercise will burn more calories and shorten your time to goal.