If you want to start losing weight, you have to start eating less. To know how many calories you shoult eat you first want to know how many calories your body burns on normal activity every day. This one can't be calculated with accuracy, but it can be aproximated based on data retreived from other people.
The number of calories burned every day depends on many factors like: metabolism, activity level, genetics and many others.
Since you can't figure out the real value you shoul get some approximate ones based on your age, weight and genre.
Use an online calorie counter, which is also known as BMR ( basal metabolic rate ). Enter your genre, age and weight and you will get the estimate.
To lose 1kg ( 2 pounds ) every week, you must burn about extra 1000 calories per day. That goal can be achieved by doing some sport that will burn up 1000 calories, or eat 1000 calories less than your daily calories burned.