It sounds like you are in pretty poor physical condition and significantly overweight. Okay, that can be changed.

For weight loss - the hard truth is, nothing really works for weight loss except eating a lot fewer calories. 1 hour cycling at 12 mph on flat ground/no wind might burn 300 calories; that is 1 bagel, or 1 donut, or 2 slices of bread, any of which takes only a couple minutes to eat. You could ride your bike for 4 hours straight, but 15 minutes in McDonalds would undo all your work. None of us can out-ride our mouths. If you don't make a very major change in your diet, you can ride that bike for a year and not lose weight.

For improving your physical condition, in particular your heart and lungs, cycling works well. You will want to start slowly - considering how poor your current condition seems to be, you should consult a doctor before starting an exercise program. As suggested, when you do start exercising, you might start by walking and taking easy rides, don't do anything hard yet. Eventually you should ride at least every other day, preferably every day. Long rides are good, but short intense rides are good too - even just 20 minutes of hard riding with your heart rate high is good. But not yet! - right now, I don't think you should get your heart rate "high", I think that would be too risky.

I suggest that you ask about this in the Clydesdales and Athena forum. There are folks there who are taking exactly the same journey you are, and they may have more to add. Your topic has nothing to do with Commuting.