copyrighted music in an indie film (mo)

I am producing an independent film, and everyone has a different answer to who I must get permission from to use copyrighted music. Do I need the person who owns the copyright, the publishing company, the artists themselves, or all of the above? Also, I have heard you must go through the Harry Fox Agency to use copyrighted music. Is this true, and how are the royalties taken care of if the film were to be bought by a major company?
I can make the movie, I just don't know about the music.
Please help me!