Maz - it depends on the current weight, but as a general rule of thumb, women should have a net calorie intake of 1200-1400 calories. This means that you could eat say 1600 but you'd have to burn off a few hundred.
For men, I think the net is around 1800-2000.
It all depends on the individual though. I cannot lose weight unless my net calories are 1200-1400. Women are supposed to maintain on 2000cals a day, but if I ate that I would be the size of the house. My metabolism is shit house! I had an eating disorder for 4 years (bulimia) so I think that was what did it to me...that and the fact that i'm not genetically blessed.
Its also important to remember that you need to leave room to increase exercise and drop calories.
If for eg, you jumped straight into eating 1200cals a day and exercised for an hour everyday, you'd have a big weight loss for the first 3 or so weeks, then you would plateau. Now, with a plateau, you need to up your exercise or decrease your calories, but on the above program you wouldn't be able to drop your calories (everyone should eat a minimum of 1200 cals a day) and increasing your exercising above an hour a day would be hard to maintain.
So, it would be a good idea to do something as follows:
Week 1 - 1600 calories, 30mins exercise 4 x a week
Week 2 - 1600 calories, 30mins exercise 5 x a week
Week 3 - 1500 calories, 45 mins exercise 4-5 x a week
Week 4 - Repeat week 3
Week 5 - 1400 calories, 1 hour exercise 5-6 x a week
And so on. This is the best way to avoid a plateau. So many people make the mistake (and I have done this in the past!) of going hell for leather straight away then they hit week 3-4 and haven't lost a thing...get disheartened and go back to old ways.
So a gradual change is the best thing so that your body doesn't get used to the same old thing happening.
Changing your exercise intensity from week to week will help to.
Eg - start out with walking, increase this to faster walking with spurts of running, then you could add other things into the routine such as gym equipment, take gym or martial arts classes, swimming etc. Adding something new in each week, even if you only do something different 1-2 x a week will help.
Sorry for the big ramble!