When I started pet sitting this is what I did:
Listing on Google Maps - They're free and most people google for businesses in their area or services in their area. If you need help with this just ask!
Advertise for free online. PM me and I can email you a list of really great sites that always give me a large number of clients - down side is you do have to renew the ads.
Get a reliable Day per page diary - preferably one of A4 size - Trust me you'll need the room!
Don't worry too much about a website for the moment, let that come along a bit later. I went through vistaprint for mine and I am currently waiting for business cards and fridge magnets to be delivered
Make sure you have an answer to GSDs4Ever's question - this is VITAL to your success, also ask yourself this question: Why should someone choose you over everyone else?