Once you make the decision that your dog needs to lose some weight, you can make a few simple life changes for your dog, and add a few happy years to his life.
Just like with humans, and virtually every other animal, weight gain is caused by excess food and not enough excercise. You may be feeding your dog too much, or feeding him a food that is too rich in carbs and not enough protien or fiber. There are many diet foods that you can put your dog on, so that he feels just as full but isn't getting as many calories. If your dog is on a high quality food already, you should probably cut down on how much you feed him. There are many guidelines on the dog food bags or online that you can follow. Another healthy trick that I particularly like is to substitute half of your dogs food with raw green beans, which are very filling, high in fiber and extremely low in calories. Your dog will get half the calories as usual without feeling starved. Also hold back from giving extra treats and table scraps when your dog is dieting.
Excercise is the second key to weight loss. If I could give only one guideline to new dog owners it would be to set up a daily excercise routine and stick with it your dogs entire life. Plenty of excercise is a vital part of your dogs life, it is important for his health, both mental and physical, and makes training them ten times easier.
Depending on the dog, you may only need to provide two twenty mintute walks a day at a minimum, or for high energy dogs you may need to provide hours of high intensity actives such as swimming, fetch, running etc.
if you do not have time to give proper exercise, find a local dog walker, play some quick fetch, or get a treadmill. Just make sure you start slow and don't push your dog too hard to quickly.