You need to add details of everything you eat (which often means weighing food) and it then calculates how many calories you are eating. It will also tell you how many calories you should be eating so you know what your target it. You then need to stick within the target for calorie intake.
It costs about £11/12 a month, but I thoroughly recommend it! It's a very good site - all about sensible eating and also has a members forum - and they are very encouraging. I think they still do a 3 day free trial, so it's definitely worth a look. You may think you know how many calories you are consuming but it's surprising how much you underestimate. Give it a go!