Go online or ask your friends. Ask your editor. Your agent will certainly know a few.
After making contact, ask what services they provide, how long you must contract for, and what it will cost. In a previous post, I pointed out the wide diversity of pricing. Paying more money doesn't always mean better service or coverage. It may mean some PR firms charge more because they contract with writers who willingly pay their fees.
Ask their specialty, even though most of them refer to themselves as full-service firms. One PR firm does nothing but radio, which is shrinking. In a town of 250 people, a radio interview might not amount to much coverage. So having 30 small-town radio stations may not be as effective as 12 metro areas. (I write may because it's difficult to quantify.)
Another PR firm does radio and TV (and the TV market is also shrinking).
A few offer blog tours. That is, they help secure interviews online with bloggers. Most of those I know write their own questions for you to respond with the answers and they display them on their blogs.
In case you're wondering, I didn't include book signings. Unless you arrange some special event along with it, you're already well-known, or you have some way to widely promote the event, most signings in bookstores have little effect.
You might hire a publicist.
The good ones can provide marketing opportunities.