How to set the ringtone in iphone?
Settings > Notifications > Phone > Notification Sound
If you're asking how to set a ringtone form a song (generally mp3) already on your phone, that's a different process.
I iTunes, "Get info" on the song, change the start/end time to the period of the song you'd like (generally 30 seconds). > Ok > Right click > create ACC version > follow prompts.
Then find where the file saved, change the extension from .mp3 to .m4r (you may need to "show extension of known file types" before you can change). "ok" then drag & drop the file back into iTunes. Ensure "sync ringtones" is selected when syncing your phone.