To lose 20 lb. in 12 weeks, you need to lose a little over 1.6 lb. a week. This is a healthy weight loss, particularly if you are already quite heavy. If you weigh 150, it will be harder to lose this much than if you weigh 250. This is not to say it's impossible, though. It just takes commitment and willingness to reduce calories, eat the right foods and exercise regularly.
Calculate how many calories you consume on an average day. The best way to do this is by writing down everything you eat and drink over a week or more, then finding an average from your set of figures.
Create a calorie deficit of between 500 and 1,000. This can be through diet and exercise; cutting this many calories from your diet alone will likely leave you hungry, while exercising enough to burn this much will likely leave you exhausted. This is the reason for combining the two.
If you create a deficit of 1,000 every day, you will lose 24 lb., while if your daily deficit is 500, you will lose 12 lb. This is why you need to try to find somewhere in between.
Speak to a personal trainer about a resistance training program. Resistance training (or weightlifting) breaks down your muscles, and your body needs to divert calories to building them back up. This will speed up your metabolism and make your body more efficient at burning calories when you're performing other activities, such as watching TV or getting the mail; these calories will go toward your 20-lb. goal.
Exercise at least 30 minutes a day, whether it's a brisk walk, a run or your weight training regime. The most effective form of exercise for fat loss, though, is interval training, in which you work as hard as you can for a short period, then work less hard for a short period and repeat this routine for around 20 minutes. Like weight training, this speeds up your metabolism and burns a lot of calories.
Reduce simple carbs in your diet that come from things like white bread, sugary drinks and alcohol. Replace white bread, pasta and rice with a brown, whole-grain variety and eliminate sugary drinks and alcohol. Your body is more efficient at processing complex carbohydrates from whole-grain carbohydrates, and can do "more with less," leaving you feeling more full from fewer calories. Fewer calories mean weight loss.
Eliminate fast food and processed, pre-made meals from your diet, as these are high in unhealthy fats your body will store rather than use. They are also the converse of the complex carbohydrates -- you will not feel as full after eating foods like this, even though they have a lot of calories.
Reduce your portion sizes to help create the calorie deficit.