Log your food (measure everything and record everything). Find out how much you eat in calories now. Subtract about 500 calories from that number. Use that new daily number (after the 500 subtracted = new number) as a goal. Log your food every day after measuring and see if you can hit that target.
That's the basics. But you should use other tools that use your weight and height to figure out your 'number of calories' - fitday has those tools!