I used to be overweight, but since then I've most more than approx. 10% body fat (hopefully dropping into the 13% range within the next few months) and now I'm really into the whole fitness thing. I can only speak for what worked for me, so you mileage may vary, but really it should work for everyone. Other posters here seem to have already covered most of the diet advice, so I'll focus on the physical exercise aspect. Note that I'll say losing fat and not losing weight. The scale can be deceptive because you'll be putting on muscle and there are a number of variables which can skew the results. I've found a much better way of measuring your progress is to keep track of your waist measurements. Make sure you do this consistently on the same day of the week at the very beginning of the day when you haven't eaten, as the waist can look bigger after you've eaten.
As others have said, the most effective way to lose fat is to mix weight training with cardio. The weight training makes your muscles bigger (obviously), which in turn means they require more energy throughout the day, meaning that less of the energy you take in (through food) turns into excess fat. In terms of the weight training, you might want to focus more on compound exercises which work larger/more muscle groups. Examples of these include (but are by no means restricted to) squats (which work the large muscles on your legs, back and torso), deadlifts (mainly back and legs, depending on the type of deadlift) and chest presses (upper body: chest, arms, shoulders). You can google which exercises are good compound ones, and focus on these ones first before moving on to more specific ones like bicep curls or calf exercises.
In terms of the weight training itself, for beginners it's generally recommended that you do 3 sets of 8-12 repetitions each for each exercise. After each set, rest for around 1 minute and then go again. There's lots of basic weight routines for beginners on google. I'd also recommend keeping logs of how heavy you're lifting for each exercise, that way you'll know you're getting stronger, even if your muscles don't look visibly bigger.
One of the most important parts of weight training is to make sure you rest. After you've worked a muscle group (e.g. chest), you need to rest those muscles for a at least a day (usually more than that, especially if you're a beginner). Basically, when you work these muscles, you break them down and tear them just a tiny bit, and during your rest period, they're repairing, much ends up making the muscles stronger and bigger. If you don't rest, they can never repair and thus never get bigger/stronger. That isn't to say you have to wait days before your next weight workout, however. For example, if you work your chest and triceps on one day, the next day you can work your back and biceps while the chest and tricep muscles rest. That way you can continually work different muscle groups while letting other ones rest.
Now onto the cardio: I'm a big believer in high intensity interval training. Countless studies have shown that it's one of, if not the most effective way of losing fat, while still preserving muscle. That is going at a high intensity for short periods of time, and then resting for a short period, then going hard again, rinse and repeat. For example, if running is your chosen sport, interval training would be running hard for 1 minute, then walking/jogging for another minute, and repeating that cycle. The intensity you should go at during the high intensity periods varies from person to person, but as a rule of thumb, you should be breathing fairly hard, and should certainly be going at a pace where you are unable to talk. Just experiment and see how fast you're able to go. High intensity interval training can be done with pretty much any sport: swimming, running, cycling, etc. My personal chosen sport is boxing. Make sure you find one that you enjoy, as you'll be a lot less likely to give up a few weeks in. I'd recommend doing cardio at least 4 days a week and at least for half an hour.
Now for some general stuff. If you've followed my advice above, you're probably going to experience soreness in the first weeks or so. A lot of soreness. But you just have break through it and keep going, and as your body adapts, you'll be getting less and less sore after workouts. The hardest bit is the beginning, and most people commonly give up their routine after around a month or so. So keep at it and after a month or two, you'll know that you've probably gotten through the worst of it, and by that time, it will have changed from becoming a routine to a way of life.
Also, don't expect the change to happen overnight. At the very least, depending on how overweight you are, it will take a few months to see some really visible changes. And as you'll likely see yourself in the mirror, you won't notice the changes as much. But trust me, you'll be getting slimmer and you won't even realise it. A good thing to do is to take a picture of yourself in the mirror in your underwear now, and then you'll have a good comparison to make when you look in the mirror a few months from now.
If you have any problems or if there's anything you don't understand, feel free to PM me.
