Dear *insert name here*,
I've heard recently you have been using the computer too much for your own good. I am writing this letter to you to tell you that it is bad for your health and you should really cut down on the use of it. It will make your eyesight bad and give you a bad posture.
I've shown you the bad effects of the prolonged use of the computer. It is up to you to change. I hope you will think through this carefully.