That has gotten somewhat complicated.  Pandora used to take all comers, but since they have gotten reasonably popular they now highly curate their selections based on criteria only they know.

The process begins by filling out a form at submit.pandora.com.  From there, you will be asked to upload a sample track.

This sample track will be the basis for whether they accept or reject your album.  If it is rejected, they will not give you any further information as to why.  If you are accepted you will be provided with further instructions regarding submitting the remainder of your album.