Did you purchase the ringtone or make it yourself by taking an excerpt from a song?
If taking an excerpt from a song, it must be 30 sec or less. Once you get the 30 sec clip, you have to create an AAC version of the clip. Open the folder with the AAC version of the clip and change the file extension to .m4r I believe. Then double click the new file and it will automatically be loaded in the "tones" folder. You can then open the "tones" folder in iTunes and drag it to your iPhone which is connected to iTunes on your computer.