The first step in becoming certified in pet training is asking yourself whether you are willing to invest the time it takes to learn the trade. Pet training requires learning the basic theory of training, plus building hours of hands-on experience. Also, unless you have a strong love for animals and the patience to deal with them, you shouldn't start a career in pet training. It takes an average of four years of intensive study to become a good trainer. Add to that the years of practical work, and you'll understand that this is a serious profession that requires commitment.
If you're sure you have what it takes to pursue a career in pet training, start by reading everything you can about the subject. Try to find information on different schools of thought so you get a variety of opinions on the different methods available. If you have a dog, start by learning the basic skills of dog training, even if your ultimate goal is to train other animals as well. Subscribe to magazines and buy books on the subject to see what's out there.
If there are pet training classes or seminars available in your area, enroll in one. The best ones cover animal psychology in addition to practical training methods. If you can't afford to pay for classes at the beginning, ask about volunteer opportunities. Many schools offer non-paid positions in exchange for allowing you to take the classes.
Before you join a school, ask to observe a few lessons to get an idea of how the school approaches discipline. Never join a school that advocates physical violence as a disciplinary method. Using choke collars, screaming, and hitting or pinching are not acceptable methods of pet training, no matter what a trainer may tell you.
One word of caution about correspondence courses in pet training: there are many out there who promise to certify you in pet training in just a few weeks. While it is possible that they offer interesting reading material, keep in mind that it's not possible to become a pet trainer long-distance. The most important part of studying to become a pet trainer is the interaction with the animals. No long-distance school can offer you that.