Teenagers concern nowadays - weight issue

Dear ladies and gentleman,
I want to present my speech about one of teenagers concern nowadays -weight issue
Many people are concerned about their weight especially teenage. They do not want to be overweight as it affects their health and their appearance, so they go on diet. Diet is all the food we eat each day. There are 7 classes of food. There are carbohydrates, proteins, fats, water, fiber, vitamins and minerals. We need all of them for our body. A balanced diet contains the right amounts of the 7 classes of food to meet the daily requirements of the body. People who do not eat a balanced diet can suffer from malnutrition and deficiency diseases.

The healthy way to avoid from being overweight or obese is to eat a wide variety of foods according to the relative amount in food pyramid. We should take 6-11 dishes from carbohydrates, 3-5 dishes vitamins. In protein class, we only need to take 2-3 dishes while in fats; we only can take them in small amount. People who have high blood pressure should avoid food that is salty and rich in fats. Continuous lack of calcium in a diet may lead to osteoporosis. Diabetics should reduce sugar and fat intakes in their diet.

I want toToday, I am here to present my speech about one of teenagersthe concerns of teenagers nowadays, which is the over weight issue

Many people are concerned about their weight especially teenagers. They do not want to be overweightlook obese as it affects their health andtheir appearance (give more focus to this point as teenagers are more concerned about this)and health , so they go on crash diets.

You need to set up a better link between these two sentences. The diet you talk about in the first sentence is a little different from what it means in the second one. I think you should avoid the second line and connect your theme better with the next line;

Also I like Pahan's suggestion;
They do not wish to look obese as they are very much concerned about how good they look. So they tend to go on crash diets without having much awareness on how adversely such actions might affect them. What they need to know is that they should have a proper diet that contains all seven nutrients in right proportions that ensure a slim fit body for them.

Even though this typescript is required for speaking of speech, putting commas between phrases is a must.
Tips for commas: read one of your sentences aloud and see where you would naturally pause, where you would draw a breath. If it's a short pause, like that just was, you probably need a comma.