1. You shouldn't eat 800 calories a day. I don't know how old you are and how much you weigh or how tall you are but 800 calories is not enough. You need 2000-2500 calories a day to maintain your weight, if you want to lose weight you should cut it down to about 1500 calories a day, NEVER LOWER. Your body can't function on 800 calories, it will go into starvation mode and cling to every ounce of fat on your body and you won't lose any weight because your body needs it to survive. I know it sounds like a good idea to eat so little calories because you think it will speed up your weight loss, but eating only 800 calories isn't healthy. Increase your calorie intake to at least 1500 calories a day to lose weight the healthy way.
2. Don't go overboard with exercising - start off slow. It depends on how active you were before you decided to lose weight, but you shouldn't suddenly do such a big amount of physical activity out of nowhere. Stop when it hurts to much and take one day off every week to allow your body to recover.
3. One week isn't a really long time, you can't expect to lose 10 pounds in the first week. Losing weight takes time and patience, it won't happen overnight. Give it time and don't stress yourself so much over it.
4. Don't take any pills, they won't help. Do it the natural way with a good diet and exercise, those are the only ways to lose weight and everything else is just a waste of time.