Dear Mary
I'm sorry to hear that your parents disgree your watching TV.Although you like watching TV, your parents think it a waste of time. you had an arguement with them because of it.
Firstly, I suggest you talk about it with your parents. let them know your plan of watching TV,studying ,doing sports and so on. you should tell your parents how much you will spend on TV. If so, they will not be worried about you.
Secondly, do more exercise and more outdoors activities, it is good for your health. Watching Tv for a long time will do harm to you.
Hope you will get along well with your parents.