Why can't I send a song from my imac (yosemite) itunes 12.1.2.27 to my iphone.4s

I am using yosemite, and itunes 12.1.2.27, I am trying to create a song and move it from my itunes library to my iphone 4s to create as my ringtone, I have tried everything no luck I must have spend 4 hours on the net trying different things. I keep getting this message.................I'm a man m4r could not be copied to the iPhone Anthony's iPhone because it cannot be played on this iPhone. I need an idiots guide, the general preferences in the 12.1.2.27 are different to the last itunes so when I watch any vidios on how to do it I can't follow the instruction, well only so far. I would appreciate any help on this.