(I'm not being horrible Kenty so please don't take offence hun!)
I feel that the exercise and calorie counting Kenty has mentioned is a bit excessive (Sorry)
At the gym i tend to burn about 300-350 cals per 30 mins, i will usualyl do about 45 mins cardio and burn about 450 cals and then i'll do about 20 ish mins of weigh work, I don't know how much i burn then but i don't really count anything.
I've done a few different 'diets' on one of them you were told to eat 1200 cal for the first week and try and stay around 5% fat so 5grams per 100g.
That gives your body a kick start, the following week cals are increasted slightly, i think i was on about 1350.
I dont calorie count tho, i point count lol
no idea how many cals i eat now, but i loose weight!