First you need to know your BMR- Basal Metabolic Rate [The amount of calories your body would burn if you did nothing but rest all day]. This figure is the amount of calories you need to consume to maintain your present state.
To calculate:
Women: 655 + (4.35 x weight in pounds) + (4.7 x height in inches) – (4.7 x age in years)
Men: 66 + (6.23 x weight in pounds) + (12.7 x height in inches) – (6.8 x age in years)
E.g. Laura 655 + (4.35 * 130) + (4.7 *63.3) – (4.7 *19)=1428
Activity level [How much you burn doing other things e.g. excercise )
E.g. Laura burns 1000 calories through walking and going to the gym
To maintain her current weight[130lbs] she needs 2428 calories daily
To lose 1lb/ 0.4 kg a week= Current Daily calorie intake- 500
To lose 2lb/0.9 kg a week= Current Daily calories intake – 1000
E.g. Laura wants to lose 0.9kg weekly, so she needs 2428- 1000 = 1428 calories a day
Laura needs to alter her nutrition in line with her goal.
E.g. Muscle gain and fat loss
There are applications that calculate your BMR for you and make it easier to keep track of your nutrition and fitness goals.
I use MyFitnessPal- Best thing since sliced bread!