iTunes if if you want to own it.
Spotify if want to try tons of music and pay absolutely nothing. It is invite only right now but if you do a bit of looking an invitation isn't hard to find.
No viruses exist in the wild that can run on Mac OS X, and there never have been any, since it was released 10 years ago. The handful of trojans that exist can be easily avoided with some basic education, common sense and care in what software you install:There is also frostwire which you can use to download music on as well, but beware of the risks as there is the possibility that some of the files could have a virus.