One way that you can lose weight, build LEAN muscle, and help your asthma, is yoga. Yoga is an aerobic workout that teaches you to slow your breathing even when you're working hard. It also incorporates flexibility training - which helps you build muscle, but avoid bulk.
Also, there are a few things that most people do wrong with their diet.
1. No soda! A single soda can contains an average of 40 grams of sugar. That's equal to 10 packets of sugar (single serve, like we use for coffee). Each packet of sugar is 15 calories... That's 150 calories per can. If you consume just one can a day, that's 54,750 calories a year. There are 3500 calories in a pound - so one can a day adds a little over 15 lbs. each year.
2. No milk! We are the only creatures in nature that drink milk past the age of baby. We are also the only creatures to drink of another animals milk - milk from cows. Milk that nature made specifically to bring a baby calf up to a full-grown cow in 4 months flat... That's a lot of weight gain.
"But how will I get my calcium, and Vitamin D?" Take a coral calcium supplement. And for Vitamin D - a little history for you. Back before there were child labor laws, kids in our country were leaving for work before the sun came up, and going home after the sun was already down. We soon began to see major health problems - reason being, these kids were getting no sun. 10 minutes in the sun each day (without sunscreen, so no more than 10-15 minutes, to avoid sun damage) is how our body naturally gets it's vitamin D. Since the kids weren't getting any sun, they started fortifying milk with the vitamin.
3. Rather than 3 meals a day, try eating 5 small meals a day. The total amount of food should equal what you would normally consume in 3 meals. But by breaking down how much food goes in each time, your body has a much easier time metabolizing the food you eat. Think of it like cleaning your room - it's so much easier to pick up a few things each day, than it is to only clean up once a week.
4. Drink lots of water. 8 glasses a day (or, an easier way is to consume a little more than 3 20oz. bottles). Water helps your body to flush out the fat that you break up during exercise.
5. Take a liquid B-vitamin supplement once daily. B12, specifically, is what your body uses for metabolic energy. This is one of the main ingredients in energy drinks - you'll get the benefit of these drinks, without all the unhealthy side effects. *note - don't get over-zealous about the metabolism thing, and take more than is recommended. You can wind up overdosing on B12, which can have negative effects on your long-term health.
If you combine the yoga with these simple diet changes, you should notice a very positive change. Also, yoga helps to regulate the production of Serotonin and Dopamine (the happy chemicals) in your brain. As these chemicals are produced, you will have increased metabolism, and a much more positive outlook in general. Having a positive outlook (not just telling yourself you need one, but actually being able to produce the chemicals you need to just FEEL that way) is a major step in the direction of looking and feeling better.
One final thought - as you are trying to slim down, throw away your scale. It's not about pounds, its about your body fat percentage. Muscle weighs more than fat, so when you're trying to "lose weight", often times you will find yourself bummed out, standing on a scale, and wondering why you've actually gained 5 lbs. So instead of putting yourself through that, let your wardrobe do the talking - get out your 3 favorite outfits, whether they fit right or not, and take pictures (1 from the front, 1 from the side, and 1 from the back) in each. Do this each week, and let these be your "scale". Make sure the lighting is the same - take the picture in the same place each week - and make sure that the distance from you to the camera is the same also. This will help prevent optical illusions that can lead to false information.
Hope this helps!