If you're looking at putting someone else's music into the game, then the first step is to reach out to the artist (or the label, if the artist is signed). You'll need to have a legal contract with the artist and the artist will have to consent to their music being uploaded to the Rock Band Network. The artist will also need to send you the individual stems. Your next stop is http://creators.rockband.com/docs/Main_Page, where the Rock Band Network documentation will explain charting, uploading, playtesting, and peer review. Getting involved on the Rock Band Network forums is also extremely helpful, as the RBN has a strong, positive community that is extremely open to giving new authors help and advice.
If you're looking at putting your own music in the game, you'll need to make sure you have the individual stems. If you want someone else to chart the music for you, then you'll want to reach out to one of the Rock Band Network authoring groups. A community-maintained list of authoring groups can be located here: http://creators.rockband.com/docs/Authoring_Groups. Otherwise, your next stop is http://creators.rockband.com/docs/Main_Page, where the Rock Band Network documentation will walk you through the process of charting, uploading, and playtesting your songs.