pet sitting is more complicated then you think. i do it for friends and family, people i know. However i was looking into expanding to a small business and its just more then i can handle right now. You need to be insured and bonded. You need contracts and legal documents so that if anything happens with your clients home or pets that you honestly had nothing to do with you can't be sued. If your really interested in starting this maybe try looking for a bigger company in your area and see if they will hire you. This way you can see how they do it and learn what you will need if you want to embark on your own.
You can do really well in this business, and you'll love doing it but make sure you protect yourself and your clients. THere are some pretty crappy people out there that will sue for things that you didn't do or couldn't prevent.
check out this book:
http://www.pattimoran.com/detail.aspx?ID=11
its an excellent source of information and can be purchased at B&N or Boarders