The best time to get photos at the zoo is right when they open: the animals are most active and the light is usually the most interesting. Try to shoot with something at least 200mm and use large apertures to blur the background and isolate the subject. Doing this will have the added benefit of blurring the fence between you and the animal to the point of it disappearing most times. Always try to shoot from the same eye level as whatever you are shooting and keep the eyes in focus. Sometimes you may have to stay at one exhibit for 10-20 mins before you really get a shot worth keeping; so don’t be discouraged.