Watching T.V. is not bad. It informs you about the world, you gain knowledge through news, programs, etc. But watching T.V. in excess can be unproductive. Watching it for long hours can be bad for your mental and physical health. It slows down your brain and nervous system. It makes you lazy and inactive which reduces your chances to be unproductive. You strain your eyes and decrease your visibility. The rays of the TV screen are harmful for your skin. Such behaviour promotes an unhealthy lifestyle and can lead to various diseases at an early age.
Devoting all your pastime to a single activity such as watching TV is unhealthy. Instead, divide your time among various activities like outdoor games, reading, drawing, cooking, etc. Something that requires physical work but at the same time makes you creative and productive.
To encourage someone to take up more activities, you can motivate them by joining that same activity with them. Doing things in a group can make you more likely to do it. Sticking to a routine by devoting a particular time to only that activity. Doing the activity willingly. Unwillingness will hinder your joy and pleasure in doing that, so make sure you are willing to do it. Find that activity which you would enjoy.