I've lost about 53 lbs since last summer, and this is what did the trick for me:
1. Cardio is your friend. Running (long slow distance ~10k and tempo runs ~sub-25 minute 5k runs) and elliptical sessions. This vastly improves your cardio-vascular fitness and gives you almost a full body work-out. With all my running sessions combined, I now run between 40 and 50k every week. If you are a big guy like me (I am 6'3, and weighed close to 290 lbs when I started), then start out with the elliptical only for a few weeks so you save your knees and other joints from the impact of running.
2. Circuit training/HIIT. When supplemented with cardio during the week, this is a great way to eventually boost your resting metabolic rate.
3. Strength training. 2-3 times a week spend about 30 minutes doing various exercises with free weights.
4. Core workout. These are usually killers but will help your performance on the field tremendously in terms of balance, speed, muscular coordination etc. Do crunches, sit-ups, plank and the side-plank.
#1,2 are targetting weight-loss and #3,4 are to make sure you dont lose muscle with all the cardio you would be doing.
Remember, its not the weight that matters most (although you are bound to lose weight with this routine), but how you feel and the inches of fat you lose off your body.
It also helped me to figure out exactly how many calories I needed to maintain my weight back then (290 lbs), which was about 3200-3300 calories each day, and I strived to make sure that CALORIES NEEDED TO MAINTAIN WEIGHT > (CALORIES CONSUMED - CALORIES BURNED).
This is relatively easy to do. There are websites that gives you close approximations of these numbers. If you have a smartphone, there should be an 'app for that'. :-)
As for diet, its just common sense really. Cut out high saturated fats like Dan said. However, some fats are healthy and should remain in your diet such as olive oil and mostly fats that you would get from seafood. Cut out deep-fried stuff completely.
Good luck!!