20 pounds in 1 week is not a realistic amount to lose...Even on the TV show the Biggest Loser, where there are 2 professional trainers drilling the contestants into a puddle of their own sweat, they rarely go over 15 pounds lost for the week.
All I can tell you is to adopt a diet of no less than 1,200 calories/day and do 1 hour of cardio/day to burn an additional 500 calories each day.
There are other diets which would have you on 500/day such as HDI, however that diet is going to require you get shots from your doc...
Personally, I think more realistic goals are the best solution...1-2 pounds/week is a good figure to aim for...if you have more weight to lose, it will be possible to lose more than this...the bottom line is to just get started with a proper diet/exercise program and realize there are no quick fixes.
