Open the iTunes Store app on your iPhone
Tap More
Tap Tones
Browse or search for a ringtone
Tap the price
Tap Buy Tone
Enter your Apple ID password or use Face ID/Touch ID
Open Settings
Tap Sounds & Haptics
Tap Ringtone
Select the downloaded ringtone
Use GarageBand to create a custom ringtone
Import or record the audio in GarageBand
Trim the audio to 30 seconds or less
Tap the Share icon
Tap Ringtone
Name the ringtone
Tap Export
Open Settings
Tap Sounds & Haptics
Tap Ringtone
Select the new ringtone
Use a Mac or PC to create a ringtone file
Convert the audio to M4R format
Sync the ringtone to your iPhone using Finder or iTunes
Open Settings
Tap Sounds & Haptics
Tap Ringtone
Select the synced ringtone