Some people will tell you can lose 20 pounds in one month, some will say you can't, many will say it isn't healthy or beneficial. . .
. . . but what everyone can say is if your seriously that dedicated than you can start eating really clean and exercising regularly and see the best results that is healthily possible in that time.
As to exactly what you should do for exercise and food, just look around this forum there are tons of threads that go into all kinds of levels of detail on eating/exercise, what, when, how much. Some starters would be:
1. Find an online calculator to figure out about how many calories you need to be eating to maintain your weight and to lose weight.
2. Eliminate all junk food from your diet. Avoid eating processed foods and simple carbs. Increase the amount of protein in your diet. Increase the amount of fibre in your diet. Eat clean healthy healthy foods (whole grain products, veggies, lean meats). Drink plenty of water throughout the day (at least 2L).
3. Divide your 'meals' into smaller meals throughout the day (4-8). Eat every 2-3 hours to keep your metabolism up. Try to make these meals all about equal size and make them balanced (some protein, some carbs etc.) Try to avoid eating late at night (assuming you don't work nights or anything).
4. Do cardio exercise every day, or atleast 5 times a week. This can include walking, running, jogging, swimming, biking. Work up to 45 min - 1 hour.
5. Introduce a weight training program. Do resistance training atleast 3X a week.
6. Don't go on a starvation diet or do insane amounts of exercise everyday- this will not help you get results. Remember to get enough sleep!
If you follow these guidelines and adjust them to you personally, you will see results. Good luck!