Well, starting out the most important thing is that you do more than you were doing before, and get your eating habits under control.
The ideal situation would be to do a resistance-based exercise program with appropriate eating and drinking habits for weight loss -- any physical activity will burn calories, but heavy resistance training will also preserve lean body mass so that of the weight lost, excluding water weight the majority of it is fat. Starting out you can use your boxing bag and bodyweight exercises for resistance training, and that may be all you need for some time. For example, you might start off doing bodyweight squats in your program for a while, then once you've got the skill, strength and endurance to do 20 or so of them for full range of motion, you might start doing bearhug or Zercher squats with the bag. You can do push ups (maybe starting against a wall all inclined surface rather than the floor) and you can press the bag overhead for your chest, triceps and shoulders. You can do various hip-hinge movements (eg deadlifts), rows and curls with the bag.
If I could give you a simple, easy to follow boxing tutorial, I would. But I can't. Nonetheless, half an hour to an hour every other day of boxing drills would be very beneficial. A good boxing session includes a solid warm up and cool down, and rounds of each drill should last 2-3 min, followed by 1 min rest. Learning how to do a decent jab, left hook, right hook, cross, left upper cut and right upper cut, along with a few different evasive maneuvres, and having rounds focused on form, rounds focused on speed, and rounds focused on forceful hitting (but not mixing those three focuses together) is a good way to keep it creative and interesting without hurting yourself. It's worth noting the every punch is a precision full-body movement -- if you're just hitting the bag with your hands, you're doing it wrong; if you're swinging wildly at the bag you're also doing it wrong. Once you get the full body into the movement and do each movement with precision, you'll throw far fewer punches in each round, but you'll get your heart rate up a lot more and burn more calories without burning out.
I hope this gives you some ideas.