It was very easy to set your songs as ringtones in Nokia handsets. You could select any song from music library and set as ringtone. But it seems like it is not as simple as that in iPhone. Simply use some iPhone ringtone maker program, like this one called Bigasoft iPhone Ringtone Maker.

Bigasoft iPhone Ringtone Maker is an easy-to-use iPhone ringtone creator which helps you easily convert music like MP3 to iPhone ringtone M4R, even create iPhone ringtone M4R from your personal music/movie collection including WMA, M4A, WAV, MP3, RA, AVI, MPEG, WMV, DivX, MP4, H.264/AVC, AVCHD, MKV, RM, MOV, XviD, 3GP and so on, and transfer the new created ringtone to iPhone via USB cable for you automatically.

For more info at http://www.bigasoft.com/iphone-ringtone-maker.html