Two things:
The first is food. What you eat is going to be 90% of the battle to losing weight. Exercise is only a small part of the whole as far as
weight loss. I would suggest you start with registering at one of the free sites like fitday.com or thedailyplate.com and logging your food. Most people are surprised by the volume of food they eat - the number of
calories and the balance of
nutrition. Once you have what you eat right there in front of you in black and white, it'll be easier to see where you need to cut back and tweak your
nutrition to be healthier.
AT your weight, you could lose weight pretty easily on about 3000-3300
calories a day.
As far as exercise; since you say that you don't do any at all right now, anything you can do will be beneficial. Start by walking. 20 mins or so of walking each day will make a huge difference in your level of
fitness.
Going forward from there, asthma shouldn't be a barrier to exercise - it's just a matter of finding the types of exercise that work for you. And for a lot of people with asthma, adding exercise to their routines helps with the symptoms of asthma. As you build lung capacity by exercising, you'll be helping to control your asthma as well.
I'd also suggest looking into weight lifting. If you have access to a
gym or to some
weights, you can start with basic things like presses and rows.
But first thing is to get a handle on your eating.

That will be the largest part of the battle.