One possible version:
Dear Linda,
If you want to be healthy and happy , you need to have a healthy lifestyle . I am writing to give you some advice and I hope you can follow it .
As a young student , first , you need to get enough sleep , so you need to try to go to bed earlier every night .You should try to sleep for at least 8 hours every day , which will help you to be active and energetic at school. Second, doing more exercise is also very important .Besides , you should remember to eat more fruit and vegetables and mustn’t eat junk food. What’s more , it can reduce your stress to listen to music sometimes after studying .Last but not least , smiling as often as possible can help you stay happy and forget your trouble .
Hope my advice can be helpful to you .Good luck to you .
Your
LiHua