Hi Mel,
Monster is right, you will have a hard time finding an exact number of daily calories that you should be following. All of the equations that you will find are estimates, but they are your best bet unless you can find someone in your area that specializes in metabolic testing, specifically BMR.
You first need to figure out your Basal Metabolic Rate (BMR) which you can do here
Fitness Tests - Fitness.com by just entering your info in the BMR tool. Write down (or remember) that number.
To determine your total daily calorie needs, multiply your BMR by the appropriate activity factor, as follows:
If you are sedentary (little or no exercise) : Calorie Calculation = BMR x 1.2
If you are lightly active (light exercise/sports 1-3 days/week) : Calorie Calculation = BMR x 1.375
If you are moderatetely active (moderate exercise/sports 3-5 days/week) : Calorie Calculation = BMR x 1.55
If you are very active (hard exercise/sports 6-7 days a week) : Calorie Calculation = BMR x 1.725
If you are extra active (very hard exercise/sports & physical job or 2x training) : Calorie Calculation = BMR x 1.9
This is how many calories your body is estimated to need each day for weight maintenance. To lose weight, you need to create a caloric deficit by either eating less, exercising more, or a combination of both.
ACSM cautions that women should never go below 1200 calories per day, and men should never drop below 1800 calories per day. Keep in mind that these numbers are VERY LOW, and in my experience with clients, is often far too low and may actually make long term weight loss more difficult.
Aim for maybe 100-200 calories per day below the number that you get from this calculation, see if that works for you over a couple of weeks, then adjust if needed.
Good luck and sorry about the very long post, let me know if you have questions! Hope this helps!
P.