Search online audition listings in your area on websites like HipHopRide.com and Craigslist (under the "gigs" section). Online audition listings are posted and updated frequently, and they typically provide phone number contacts for you to call and set up appointments. You won't necessarily get to audition directly for the artist at first; special recruiters are often hired to advertise tryouts and conduct the open calls for dancers. Join a general online casting network for free, as actor and model websites also offer music video listings on a regular basis.
Network with local hip hop artists to get smaller gigs as you start out. After appearing in several lower budget music videos, you'll be able to create an audition reel that you can show to more popular acts in addition to your live auditions. Networking will allow you to connect to the larger community of hip hop artists. After speaking to several musicians and making them aware of your talent, you'll begin to hear about audition opportunities before they're publicized anywhere else, and you'll have personal ties to vouch for your skills.
Ask around at any dance collectives or dance studios you may be involved with. Hip hop instructors at area gyms and community centers often post printed notices of music video auditions as they become available, in order to keep their students informed of talent searches. Even if you're not involved with a dance group or organization, don't hesitate to ask community insiders about opportunities for an appearance in front of the camera.