The "Big 3":
Start with a goal -- Set an achievable short-term goal, write it down, and read it several times per day. Think about your goal constantly, until it becomes an obsession. This will help to "set" your mind right for fast success.
Practice clean eating & cycle carbs-- Avoid all processed foods and refined/man-made ingredients, especially white sugars and flours. Avoid all fried foods. Eat only lean proteins, fish and seafood, fruits and veggies, nuts and seeds, and beans and legumes. Also cycle your starchy carbohydrate intake to keep your metabolism boosted.
Do short, intense workouts often -- 3 to 6 days a week do a short and intense workout based around brief bursts of high-intensity activity (aka intervals). They should include full-body strength/weight training as well as high-intensity interval training (HIIT).
The other stuff:
Boost your energy naturally -- Get 7+ hours of sleep per night and do some brisk morning exercise every day. Avoid drugs and alcohol.
Try a short juice or lemon water fast -- For 1 or 2 days drink nothing but raw fruit and vegetable juices and/or spring water with fresh-squeezed lemon or lime juice in it.
Create momentum by going for small "wins" every day -- Set small goals for each day. Each time you reach all your goals in a day mark it down on a calender. See how many days in a row you can make it. This will build powerful momentum very quickly!
Use the right supplements -- Use whey protein, omega 3 fat supplements, and a good multivitamin. These are the only supplements you need for fast and healthy weight loss.
That's it: everything you need to know to lose 15 to 20 pounds fast!