Can someone help me to fix my grammar ?Make my paragraph to better.?

I think exercise improves fitness and increases the body's ability to protect itself. Exercise also helps to enhance the body's immune system. A little exercise increases blood circulation and strengthens the heart and lungs. I think exercise helps me to relax. When I finish exercising, I feel relaxed: and my muscles too. I have heard that, if you are young and want to grow taller, the best thing to do is to exercise. I do not know if that is true or not.