Hi LA
A good place to start is to calculate your Basal Metabolic Rate. This gives you an idea of how many calories your body needs just to exist (if you just lay in bed all day).
If you were to consume the amount of cals on the BMR calculator each day then just by walking around and doing some additional structured exercise (swimming, cycling etc) then you'll create a deficit. You need to create a 3500 cal deficit to lose 1lb (i.e 500 cal deficit each day will get you a 1lb loss in a week and so on)