Dear Miss Snark,
You mention that a person should never send a query to someone that they don't know. Since I don't know any agents or publishers, what's the next step? My book is complete, my synopsis is written and my query is ready for viewing. Please help.
You didn't get your invitation to the Meet the Agent Winter Frolic at Miss Snark's Ice Palace, wherein everyone meets Miss Snark and we all "gather 'round" reciting first chapters (simultaneously of course)?
ok, ok, I'm joking.
"Know" in this case means you see evidence of business activity beyond a glitzy web page or a solicitation from the agent to bypass the rigors of the slush pile.
"know about" is probably a better way to express that.
You can "know about" an agent by logging onto AgentQuery.com, by flinging yourself into the depths at AbsoluteWrite.com, poring over Preditors and Editors; by perusing Publishers Marketplace, by ogling Publishers Weekly (your library has a copy even if they don't have it on the periodicals display--you can ask for it); by reading blogs like the ones on my blog roll.
You can actually meet real live agents at writing conferences too, but you don't have to meet them to "know about" them.
The advice to not query agents you don't "know about" is to help you avoid agents who haven't sold anything and charge you money. It's business advice, not a social nicety.