It's a great way of advertising your business. Be sure to put that you are a member of PSI on your business cards, as pet owners recognize PSI as a highly professional and recognized Pet Sitting organization. You will have to then think about the sort of animals that you would like to service. Some pet sitters provide services to cats only, while some pet sitters will provide services to any pet. Also, some pet sitters only provide regular daily services, such as dog-walks, while others prefer to provide occasional services for people who go out of town occasionally, or on vacation.
Many pet sitters do both.
Figure out what you can handle. The next and most important step is writing a contract. In your contract you must list everything about your service. Your rates will be the most important thing to consider. Once you have your contract ready, make sure that each potential client has a copy.