What do you want to Photograph?
Sometimes you can catch a great moment, the lion roaring, the zebras running, the snow leopard eating or a monkey having fun. 
Most of the time, you will find an animal resting. Again, be prepared, do some research on the animals you want to photograph. 
Does the lion roar at certain times of the day, are the zebras more frisky at different times, when does the snow leopard usually eat and when can you anticipate a monkey having fun? 
Learn about natural behaviors animals have in the wild. Many of these behaviors can be seen in the zoo.
Zoos have regular feeding schedules and routines. 
Read all you can about animals you want to photograph and you may be able to anticipate certain behaviors and capture it on film.