Get a professional press kit. This includes a bio, postcard photo, CD and contact information. Have online links to your music available.
Research the club to make sure it's an appropriate venue for your act. Don't waste your time on a larger venue if you're just getting started. Know what kind of music is generally played at a venue before contacting the booking agent.
Contact the booking agent at the club before you send your press kit. A press kit that comes unsolicited to a club ends up in the slush pile, which is the No Man's Land for the undiscovered artist. You don't wanna be in the slush pile. You wanna be in the shorter pile, closer to the desk of the agent. You do this by making personal contact and asking permission to send a kit.
Follow up and be persistent. The first follow-up call (or email) should be to see if the club received the kit. The next should be to ask for a gig. Unless you receive a flat "no, we don't want you here," keep following up every 2 or 3 weeks.
Remember that persistence pays off more than talent does, unless you are extremely untalented. Even then, you might still get a gig, for kicks and giggles, just so the club booker can say, "look, you didn't bring anyone out for that first gig, so we're not going to give you another."
Don't be that act! Get paying butts in seats, even if the rest of the world thinks you suck. Prove to the club that you deserve to be asked back.