Zoo are very underappreciated these days, both by people looking to enjoy their day, and by photographers. A zoo is a great place to practice your wildlife photography. Animals don't really cooperate when you are trying to take their picture, in fact after a while you will actually start to believe that animals know what you are doing and are purposefully trying to make it difficult for you. So with this kind of blatant disrespect for you and your camera, you will need to learn their tricks and behavoirs so you don't end up with blurry pictures or sharp pictures of the animals rear end. This is where zoos come in really handy, as they allow you to practice without having to spend money on a safari or deep jungle expedition. Here are some tips to keep in mind that should help you at the zoo.
1. This is practice. The reason I say that is almost every zoo has rules that you cannot take pictures there and use them for profit. So if you are looking to take pictures of lions and then sell them, well that's not going to happen at a zoo. At least not legally.
2. Get there first thing in the morning or if the zoo stays open late, get there near sundown. The animals are more active early and late in the day and you will be able to get pictures of the bear when he doesn't look like a rug at the hunting lodge.
3. Check the zoo's webpage for events ahead of time. For example, I know a lot of zoos are starting to do events where they run the cheetahs. Showing up and taking pictures of a cheetah cruising along at 40+ MPH is always a fun experience. It's also pretty hard and helps you learn the techniques for moving targets.
4. Telephotos rule. Bring the longest lens you have. However you should keep in mind that most zoos are very nicely landscaped, so you might bring along your macro lens for flowers as well.
5. Be patient. Sometimes sitting half an hour at one pen is going to get you the shot you want. In the wild, you will be sitting at one spot most of the day and waiting for animals to come to you, so learn to be patient.
So that should get you started, just remember zoos can be a lot of fun. I used to get a year pass to the local zoo, and when I had saturday mornings where I had no plans, I would just go to the zoo for a few hours and enjoy the kangaroos, elephants, turtles, deer like creatures (seriously who knows the names to all the four legged deer like mammals?), and otters. BTW....taking pictures of otters swimming is probably harder than the cheetahs running, so good luck!