If I were you I would try to find some models to follow. If you have access to a good library with a periodicals department, you can find reviews of some TV shows in Weekly Variety. They do more reviews of movies and television than any other publication in the country, and their reviewers are experts. You can, of course, consult TV Guide, which used to be, and probably still is, the most popular magazine in the United States. If you don't subscribe to this magazine, it should be easy to get a back copy from a friend or neighbor. The copies of TV Guide that would be most useful would be those that are issued when the new TV season starts, but there should be some reviews in almost every issue. Even a good newspaper would have TV reviews which you could read to get an idea of how to write your own. A TV review should give a lot of factual information about the channel, the time, the type of show, the actors (if drama), and everything else a viewer would want to know--even if the viewer has missed the show. The trouble with trying to review a television show is that everything goes by very quickly and it is hard to watch something and write notes at the same time. If possible, it is extremely useful to get published information which you can use for factual data while being free to focus your attention on the audio-visual presentation. At the very least, you should use the TV Guide for factual information, but Daily Variety is a positive goldmine of factual information--and there is nothing unethical about using it for that purpose.

Writing a review of a TV program is much the same as writing a book review.  The first thing you will want to do is watch the program.  You may want to take some notes while you want so you can clearly remember your responses later.  It might be helpful to note things you liked, things you didn't like, things that you thought could have been done better, particular character traits, the general plot, and other details.  When you go to write your review, remember that a review is not the same as a summary.  A summary simply states what happened, but a review expounds upon that summary.  In a review, you can assume that your audience has seen the TV program in question.  Say more about your opinions than general descriptions.  You want to give an objective view of the show,  its plot, its characters, the technical direction (like lighting, framing, etc), and any other details you can.