Hiya, I use an online calorie counter, it just makes it easier for me to list down everything I've had, or will have, and to work out how many cals I have left over. I normally just do it as I go along, unless I know I'm going to have a busy day, and then I'll probably note down everything I plan on having that day for breakfast lunch and dinner, just so I know how many cals I've used.
Your number of calories to aim for really depends on your activity levels, the amount you have to lose, and how much you want to lose per week.
This online counter tells me I should be having around 1800 cals a day, and doing at least 200 cals worth of exercise to lose 2lbs per week. But to be honest I'm sticking to nearer 1500 calories a day, I find that now that I'm eating more fruit and veg it's far easier to stick to 1500 calories, I always seem to have some left for a snack in the evening!