Buy a few books that explain the basics of promotion. Read the books and take notes on how to promote a show. These books will explain how to create flyers and distribute them around your area. These books will also give you a little bit of knowledge on what to expect when you do hire a promoter. Most promoters will also for an upfront payment before they begin. Other promoters will work from commission.
Call different public relations or talent agencies in your area and ask if they have anybody who specializes in hip-hop promotions. Most talent agencies will have a scout that is looking for new talent. They may be able to get you in touch with a promoter.
Talk to club owners. Clubs do not just open with lines around the block. Clubs have promoters that they use to get people in the door. Club owners know who the best promoters are in your city. Since clubs play hip-hop music, they should know who to talk to.
Set meetings with different promoters to find out what they think they can offer you. Different promoters have different connections, different rates and different methods of promotion. Meet with a couple to verify what you think would be the best style for your show.
Get everything written in a contract. Verbal agreements are not enough when using a promoter. Professional promoters will draw up a contract before they are paid. If your promoter doesn't want to draw up a contract, find another one.