Ok you're gonna need a healthy diet, and at least 20 minutes of aerobic(cardio) exercise everyday.
Do any kind of aerobic exercise, whether it be a light jog, or a brisk walk, just make sure it gets your heart rate up.
Start off by doing 20 minutes of any kind of aerobic(cardio) exercise everday, then gradually build up to 30 minutes, then 45 and your ultimate goal should be an hour.
Be sure to stretch 5-10 minutes before and after your workout.
Try only eating lean meats like fish or chicken with some fruits and vegetables, maybe a salad occasionally.
You can have some junk food rarely, every 3-4 weeks AT MOST, and if you do make sure it's not much.
Stay away from sugars and carbohydrates as much as you can(except the ones you get from fruits and vegetables)
But just make sure you follow this one rule; AVOID SOFT DRINKS AT ALL COST.
Seriously, those are *really* bad. It's like drinking calories from a can. It'll really pile on.
If you do all of the above you should start seeing results in 2 weeks and reach your goal(assuming it's not surpassing 20 pounds in 1-2 months)
So in short, have a healthy diet do aerobic exercises and you should get amazing results. I sure did.