One pound equals 3500 calories, so to lose a pound per week you need to burn 500 calories more per day than you eat (500 cal. X 7 days = 3500). You can do this by cutting your calorie intake and/or adding exercise.
However, how many calories that means for you depends on your body. There are lots of calculators that can give you an idea of how many calories you should consume (google "calorie intake calculator"), but it seems that they give very high estimates. For example, if I input my stats (25 yo female, 213 lbs, 5'10", moderately active), they tell me I should eat anywhere from 2100-3200 calories per day! Yet through my own experimentation I've found that if I want to lose weight consistently I should eat 1300-1400.
You can use the online calculators (or figure out how many calories you eat on a daily basis and subtract 500 calories per day) to give you an estimate, but my best advice is to listen to your body. If you're eating 2000 calories or so per day now, try eating 1700 and doing some extra exercise for about a week, then see the results. If you don't lose anything, try 1500 calories, and so forth and so on. It's a puzzle each one of us has to figure out. The calculators and formulas can help, but no one knows your body better than you do.
I'm not aiming for skinny, I'm just aiming for happy.