Dear Micha,
I have a 15 year old daughter who is 5'7" and weighs about 140 lbs. So I had to answer you!
First, as you see, nobody here is giving you advice how to loose 20 lbs - why not? Because it would be very bad for you to try to loose 20 lbs, especially fast. Is this someone's opinion here or is there a more objective way to say?
To find out your BMI and what is normal, go to this Website:
http://nhlbisupport.com/bmi/bmicalc.htm, or
http://www.cdc.gov/nccdphp/dnpa/bmi/calc-bmi.htm
These are calculators where you put in your height and weight,
and it gives you your BMI. They also indicate what BMI is normal, under or overweight. I am also 5'7" and clearly can loose some more weight but I am (just barely) in the normal range.
You are in the lower third of the normal range, but your goal weight of 118 lbs puts you at just at the edge where you would be considered underweight - which is not healthy. So your goal puts you below what is good for you.
So, once we have established that you don't have a weight problem, and shouldn't loose 20 lbs, then you should really stop doing Atkins or any other diet - you don't have to drink soda or eat candies, but you shouldn't try to loose weight.
Instead, we have to figure out why you are desperate.
Which brings me to the other suggestions - talk to someone you trust - your parents, your physician, a teacher, the school counselor, or the parents of a friend. As others here said, there is danger in going onto a diet in your state of mind. Many teenagers and their parents find this age difficult - there are so many changes in body, feelings, attitudes of you to the world, of the world to you. It helps to find people to talk to, and you should.
If you really can't find someone to talk to, call one of the teenage hotlines, for example 1-800-448-3000 or 1-800-443-8336.
It might help you to talk to other girls who feel just like you!
http://chat.girlsandboystown.org/hotline.asp
But I hope you find the courage and strength to talk to a real person soon about how you feel - it is possible to feel better!