What you need to figure out is your BMR. Depending on a bunch of different factors (height, weight, gender, and age) - these all add up to your Basal Metabolic Rate (BMR) - your BMR is the amount of calories you burn just from living day-to-day, not factoring in exercise.
Using this calculator:
http://health.discovery.com/centers/...sal/basal.html
I will type in your height, weight, and I'll estimate you are the median age for the 20-somethings forum (25). Using this calculator, your BMR is: 1,575. So this means you burn 1,575 calories per day just by living.
Let's add this number up the other factors you outlined before -
So you burn (1575) calories per day as calculated + an extra (500) for exercise which equals = (2075) calories burned per day. If you intake 1500, (2075 burned - 1500 eaten = 575). You will have a 575 calorie deficit (deficit being you burned x-amount of calories more than you took in) each day if you continue with this plan. (575 x 7 days per week = 4025 calorie deficit for the week, which is a little more than a pound).
BMR explains why it is becomes increasingly more difficult to lose weight as you lose. The less you weigh, the lower your BMR is - this means if you were to keep up the same 1500 calorie diet through your weight loss, your losses will get smaller and smaller with each pound you lose because your BMR will be lower. So as you lose weight, your calorie intake will need to decrease (or your exercise will need to increase) - but as long as your deficit remains around 500 per day (BMR+exercise, you will continue losing around a pound a week.
(BMR + calories burned exercising) - (calorie intake) = Daily Calorie Deficit
Daily Calorie Deficit x 7 days = Weekly Deficit
A weekly calorie deficit of 3,500 is needed to lose one pound.
Does any of this make sense?
*One thing to note is, the BMR calculators do not factor in lifestyle (calories burned by walking to and from work/school, grocery shopping, cooking, cleaning, showering, etc.) This BMR calculator also factors in lifestyle - sedentary, lightly active, moderately active, etc:
http://walking.about.com/cs/calories/l/blcalcalc.htm
For purposes of explaining, I used to the basic calculator - but for a more accurate account of your daily calorie burn, I'd recommended checking out CalorieCount on About.com and using their sign-up to calculate your daily burn or the above-mentioned calculator. As a college student who walks to and from multiple classes a day, but also sits in front of the computer for long periods of time doing assignments and such, I classify myself as "lightly active" - just to be safe and not overestimate the amount of calories I'm burning.
With the BMR method, you can pretty accurately predict the kinds of losses you will have. Of course, your body will ultimately determine the loss for you, but BMR helps you hone in on the kinds of calorie in-takes and burns you should be achieving in order to lose x-amount of weight per week.
Phew! I hope that helps! Let me know if any of it was confusing.