Basal metabolic rate is also called BMR. It is gives you an estimate of how many calories you need to maintain your current weight. There is a formula for calculating your BMR and when you get the number after doing the calculation, you can make some changed to your weight.
If you know your BMR, then that is the amount of your calories you need to eat to maintain your weight. When you consume less calories than your BMR, then you start to lose weight.
When you eat the same calories and start exercising then also you start losing weight.
I would also suggest to consult your doctor before going on diet because it is always good to know whether you will have an issue or not.
This video on ehow gives some good information on how to calculate your BMR.