You've received a lot of good advice here about how to become a professional Musician, as M. Ted Said a career in music is not necessarily that. If you are interested in the writing, publishing aspect of the business the ASCAP and BMI websites are excellent resources. For teaching etc. I would check out websites for music schools, Universtity and private. I don't know what the orgs are in the US but there are unions and associations for sound and recording engineers, performers, songwriters, musicians etc.

What do you call a musician without a girlfriend?