The idea behind losing weight is simple: burn more calories than you take in. However, this is SO MUCH easier said than done.
But, it is a simple equation...either eat less calories, burn more calories, or some combination of the two.
Eating less calories can be almost impossible. It often takes a super human will to do that. So for me, I've focused more on the other side of the equation, trying to burn more calories.
Since you'll likely only have so many hours during the week to exercise then it is important to spend that time doing exercises that will permanently speed up your metabolism. A common resting metabolism might be, say 70 calories an hour, about 1700 calories per day. Well, what if you could speed that up to, say 85 calories per hour? That is over 2,000 calories per day! That is 300 extra calories burned.
The best way to speed up your metabolism is to build up your muscles by doing weight training. I'm not talking about becoming a body builder or anything, but I am talking about a regular regimen 3 or 4 times a week of good solid weight training. Even resting, a body that has lots of muscle will burn more calories than a couch potato.
Don't neglect your cardio exercises, these are also good, but spend a lot of quality time weight training.
I've been lifting weights regularly for over 10 years. I usually lift about 6 to 7 hours per week. On top of that I do about 3 or 4 hours a week of cardio, and I walk about 5 miles per week with my dogs. Between the calories I burn working out, and my higher resting metabolic rate from weight training, I calculate that I burn about 800 to 900 calories more per day than my brother, who is the same height and build and only a couple of years older. He almost never exercises and is about 40 lbs heavier than me.
Finally, even exercising like that, it is still important to focus on the other half of that equation, which is proper nutrition. Although I'm pretty good at exercising, I'm not exactly the poster boy for the best nutrition. But simple things I've done include soda only once or twice a week (water the rest of the time), and don't have any chocolate around the house! Good Luck!