Before people chime in, I just want to mention that this year, for the first time in about thirty years, the Macy's fireworks will be on the Hudson River (that is, the New Jersey side, yaaaaay!) rather than the East River (that is, the outer boroughs side). So the spots that have worked for years and years will not work this year.
That said, if you go to the Macy's website or a New York tourism website, there should be a map that shows viewing areas.