How to transfer a song to my Iphone 4s?

How do i transfer a single song to my iphone? I have a PC. I downloaded some songs from youtube and put them in my music folder on my computer, I have also got itunes. But i can't seem to be able to transfer a single song on it's own?

You'll have to buy the song on iTunes, and download it straight off your phone. Or you can buy it on the computer through iTunes, and download it on there, then transfer it to your phone using iCloud, or plugging your iPhone into your laptop!