How do I take a song that I bought from the iTunes store off my iPhone?

sync your iphone to your iTunes then select transfer purchases to my pc when itunes asks you then once thats happened you will want to go to the itunes directory where your music is found which is usually Music folder then itunes or depending on your operating system you may want to go to program files then check both folders for itunes and once you have found it check though the itunes folders until you find something along the lines of music then purchases or the other way around perhaps and it should be in there and take a copy of that to you desktop and boom,

Peace!