You didn't get to where you are overnight. Why do you think that it is possible to reverse the damage quickly?
One of the most dangerous things you can do to your health is to make sudden and drastic changes in your level of physical activity in an attempt to compensate for years of poor habits. Engaging in a strenuous exercise regimen without physician supervision is a significant risk factor for acute or life-threatening injuries (e.g., stroke, heart attack).
However, that is not to say you should not make any changes: clearly, you do have to break your bad habits, become more active, and improve your diet. Push yourself, but not too hard: lasting weight loss takes time, contrary to the reality TV messages that Americans are bombarded with on a daily basis. Why does it take time? Because weight loss is fundamentally about making lasting lifestyle changes, and habits are not things you can break in a month or two and consider yourself "cured." It takes years of consistency to demonstrate you've overcome them, and put in their place healthier coping mechanisms.
Diet is always the first thing to look at when dealing with obesity. Eliminate--and I really do mean eliminate, as in zero, none--all refined sugar, especially anything that contains high fructose corn syrup. You cannot have anything that has sugar added to it (aka cane sugar, HFCS, fructose, sucrose). Any sugars you do eat must occur naturally in the food itself; e.g., fruits, vegetables, milk. Absolutely NO soda, diet or otherwise. No fruit juices! Drink water instead. First thing in the morning after you wake up, drink two glasses of water. Some degree of caloric restriction is necessary, but it is difficult to do this unless you count your calories. Cutting out carbohydrates in general (aka "Atkins" or "Paleo") is not needed. You can have bread and pasta--as long as it doesn't have added sugar. Eat more fiber.
Exercising in a gym is actually not important in the beginning stages. Just walking a few miles a day would be enough exertion to get you to increase your metabolism. The mistake that a lot of people make at the beginning is that they think they need to hit the gym hard--but they burn out, injure themselves, and suffer pain, all the while ignoring the fact they're consuming so many calories they would need to exercise for 8 hours a day every day to offset that caloric surplus.
Finally, you need regular medical supervision. This process will suck at first, and you will probably feel really crappy, tired, and irritable, but that will last only a few weeks until your body adjusts. Make it past that initial adjustment period and you will find you'll have more energy, and you will have lost weight. Then ramp up the difficulty and walk longer distances, keep up your diet with the goal of eliminating processed foods, and your body will figure out that it needs to continually adapt by losing more and more weight.