I guess I'm like most people: I rode a bike everywhere as a kid, then got my first car and promptly sold the bike. Through my twenties and thirties a moderately active lifestyle and a reasonable diet kept the weight off but once I hit my forties reality (and those extra pounds) started to set in. So, I decided to do something about weight loss.
These days you can pick up a comfortable touring bike with 18 or 21 speeds very cheaply, even less if you buy a good second-hand model. Add a rear bike rack and panniers, and suddenly you have a vehicle with which you can fetch your weekly shopping and run all kinds of errands. In fact over the past three weeks I've only started my car once, and that was to pick up one of my kids from school.
If you ask most people why they haven't considered riding to work, weather and fear of cars are the two major reasons. I can't do much about the weather, except to recommend all-weather gear, but when it comes to traffic I should point you towards the very useful Google Maps. Examine your local neighbourhood with an eye for quiet back streets, bike paths and dual use cycleways/footpaths. Better still, download and install Google Earth, which allows you to draw and save routes right on the maps.
The first task is to flag all nearby shopping centres and other places you visit regularly. Next time you drive there check for bike racks where you can safely lock up your machine, and also keep an eye out for the easiest way to ride in and out of the area. (Use street view on google maps/earth to explore back streets, too.)
Over time you can build up a localised map, and if you fit an odometer to your bike you can also track the distances. I write the distances right into my path names on google earth, so I know how far each ride will take me.
My goal is 20-35km per day to maximize weight loss without spending hours in the saddle, but I started with 5-10km until I got used to it. Any excuse for a bike ride will do - from buying a stamp to depositing a cheque at the bank or trying a new butcher for something nice for dinner. Most days I pick a favourite ride and tackle it for the sake of exercise, but there's a sense of accomplishment when you use your humble bicycle to fetch food for the family, to run an errand, or to pay someone a surprise visit.
Over time I've added bits and pieces to my bike, including a handlebar mounted holder for a gulper mug (holds two cups of hot tea), aerobars which allow me to lean right forward when peddling into a headwind, mudguards so I can ride in the wet, the odometer for speed and distance travelled, a rear-view mirror so I can keep track of cars coming up behind me, the all-important bike lock and helmet, panniers for shopping, a spare inner tube, various bike tools and so on.
I've always been mechanically-minded, so maintaining and repairing my bike is a snip. But even if you're not a spanner-jockey, most things on a bicycle can be adjusted or replaced with a screwdriver, allen key or adjustable wrench. They're simple machines, the direct opposite of any modern car.
In summary I've been using my bike extensively for a couple of months now, and I've lost 5kg (about 11lb) of weight, my legs are much stronger and I feel so much better. That's 5kg of weight loss after two months - imagine what I'll feel like after a year or two!
If you're very unfit, overweight or have medical issues I would recommend a checkup or a chat with your doctor to get some advice on the idea of cycling. And I'd start by taking it really easy, maybe 3 or 4 miles at a time with very gentle peddling. The good news is that cycling is low-impact exercise, which means it really is accessible to almost everyone.
Finally, you might like to view my cycling projects:
How to make an ultra-cheap rear-view mirror
How to make an effective bike headlight - cheap!