If you want to download music legally, you can NOT download it for free. Limewire is like Kazaa, Frostwire is like Limewire, and all of them are like the original Napster. It's illegal.
If you want to download music legally, get an iTunes account, download the software, and pay for it. There are other methods of getting songs for free; I know of dozens of methods that people have told me. However, these are still illegal methods of obtaining files of copyrighted content.
If you want to download songs legally, use iTunes. (Because, while Amazon works just as well, I've been ripped off by them many times. I've a bit of bitterness towards their company.)
But, there ya go.