You can burn a brown candle that has been charge with your intentions to help you lose weight. Remember that magic can help one to achieve a goal but it can not be used alone. Mundane actions still need to be taken in order for a any spell especially this type to work. The brown candle combined with diet and exercise may result in weight loss.
Actually, if you do not eat for an extended period of time, your body will digest and break down its own fat, then when you eat, our body will try to restore the fat as soon as possible, starving yourself is the worst way to lose weight.