Don't try to do it quickly. "Quick fix" suggestions always result in putting weight back on. They're unsustainable, bad for your health, and usually just a way of depriving suckers of their money.
1. Eat appropriately. Calories in minus calories out is a good basic assessment, but try to go with that food pyramid they crammed down everyone's throats in high school. Make sure you're drinking plenty of water, too - not "8 glasses a day", that's a misrepresentation, but enough to stay well-hydrated.
2. Get some regular exercise. Don't try to break any world records to start, but keep scaling up. Do what works for you - the gym, going running, swimming, rock-climbing, dancing - just do it consistently and regularly, and increase the duration/intensity as you get fitter. Cross-training is probably a good idea once you've established a pattern for exercising.
3. Stick with the changes. This is the hard part. Don't fall back into old habits, and eventually your new ones will replace them.
OT (slightly) - getting dumped and going vegetarian seem to help; everyone I know who's had that double - myself included - has shed at least 10kg and dropped one or two belt sizes. Not eating meat helps you drop weight, but it makes it harder to put on muscle.