How do I get a coupon for my first time booking?

Each of the sitters has their own personal coupon code, and can give it to you on request.

Some sitters advertise on Craigslist, put business cards up in pet-related businesses, or similar. Their code is usually in the ad or on the back of the card.

Probably the easiest/best thing to do is run a search on the Rover website for the sitters in your area, read through the profiles, contact a sitter (or multiple sitters), and when you make your decision, ask for her or his coupon code.