You can’t; well, not in a direct way. Ringtones are special files, short audio files, marked as ringtone. They have to be under 40 seconds. They have to be tagged as ‘Ringtone’ instead of ‘Music’.
There are several apps in the iOS App Store that will turn a song into a ringtone (search for ‘ringtone maker’). Also several desktop applications can assist in this.
Or you can buy them separately in the iTunes Store (roughly at the price of a full song).