Unfortunately, some people would now argue that zoos fulfil a valuable role in preventing further climate change, habitat loss and species extinction because of the research that they do - and the breeding and release programs they operate. So if you really believe that all zoos are bad, and want to see them all closed down, never to re-open, then may be you could start with an opening paragraph based on their failure in these missions. Maybe you could say that their research is ineffective and costly on the animals' suffering and therefore the end isn't justifying the means. Of course, you would need to find good sources and quotes that you can cite in order to prove your point. Perhaps there are some bad zoos with a poor animal welfare record that you can refer to.
Here are some ideas, which you will want to choose from, depending on your own claims. I would not list them in an introduction as I am doing here. Instead, you will want to do some more research and fill in the gaps around these ideas.
Zoos: preservation or explotation?
Zoos are a waste of land resources, as they take up acres and acres of public space.
Zoos are a drain on tax dollars, as they require much maintenance, revenue for upkeep, and mass transit.
Zoos are not natural habitats for animals. Animals face many problems that would not occur in the wild, such as self-mutilation, domestication, reproductive disorders, a physiological imbalances (pacing, rocking, aggression).
Zoos are less about animal education and more about animal exploitation. They are interactive amusement parks, full of gimmicks.