Diet and exercise. They work together, don't try to do one without the other. A good diet gives you energy for exercise, and exercise makes your body naturally want to eat a good diet.

For exercise start with something really small and easy that you will do every day. It doesn't matter if it's too small to make an impact, the point is to get in the habit of exercise every day. Once you're consistently doing it every day for a couple weeks then you can gradually dial up the intensity. If you miss a day don't get down on yourself and quit, instead forgive yourself and get back on the wagon the next day. To satisfy your software developer instincts your can use an app to track your progress. On Android I recommend Regularly and FitNotes, both free.

For diet, don't try to eat less bad food. Going hungry sucks, and bad food tastes good! Instead of eating less bad food, eat more good food. For example, stuff yourself with healthy snacks even if you're not hungry. When you're served a plate with a mix of food, eat all the healthy stuff first. Filling up on good food will leave less room for bad food.

This is what works for me.