Get in touch with the band or it's management. It would help if you personally knew the band, or had any previous experience in merchandising/music/fashion, etc, too. It's not one of those jobs that you can walk in with a resume and apply for. Hope I helped :P
Generally it would help if you knew the band, because chances are, they're going to take someone they know and trust on board to be dealing with their merchandise and money.
Having a good relationship already with the band would be a good thing. They need to know that you are reliable and trustworthy. It's an important job.