Eat veggies. No fat. If only 20-60 pounds to lose, then focus on veggies, protein, and exercise. If more than 70 pounds, you will likely need to consider bariatric surgery. Either way, you need a long term transformation in the way you see food. Learn to love healthy food. Learn to be disgusted with greasy food and sugary food.
Lasting weight loss. In order to lose weight and keep it off, you need to make a commitment to a permanent lifestyle change. Most importantly you need to commit to a more active lifestyle. No diet is permanent. Eat 75-85% of your daily calorie needs (and count them). www.bmi-calculator.net/bmr-calculator/ try to lose at a healthy pace of 1-2lbs per week and most importantly keep that exercise level up. Good luck.
Surgery. Weight surgery.