Open the iTunes Store app on your iPhone
Search for the song you want to use as a ringtone
Buy the ringtone version if available
Go to Settings
Tap Sounds & Haptics
Tap Ringtone
Select the purchased song ringtone
If the song is not available as a ringtone, use GarageBand
Install GarageBand from the App Store
Open GarageBand and create a new project
Import the song into GarageBand
Trim the song to 30 seconds or less
Tap the Share button
Tap Ringtone
Name the ringtone
Tap Export
Go to Settings
Tap Sounds & Haptics
Tap Ringtone
Select the exported ringtone
If needed, sync a custom ringtone from a Mac or PC using Finder or iTunes
Convert the song to a compatible ringtone format
Add it to the Tones section in Finder or iTunes
Sync the iPhone
Go to Settings
Tap Sounds & Haptics
Tap Ringtone
Select the synced ringtone