iPhone users are able to purchase ringtone versions of many popular songs off of the iTunes Store. This process, however, does not allow the user to choose what portion of the song is set as a ringtone and is limited to the songs available on the iTunes Store. Luckily, iTunes also allows you to create ringtones out of any MP3 file if you already have the song file downloaded onto your computer in a few simple steps.
Open the iTunes application and locate the MP3 file you wish to set as your ringtone. Play the song file until you find a 30 second clip that you wish to set as your ringtone. Take note of the clip's start and stop times.
Right-click the song title and select the "Get Info" option from the context menu. Click the "Options" tab and place a checkmark next to the "Start Time" and "Stop Time" options. Enter the start time and stop time you noted in the spaces provided and click the "OK" button.
Right-click the song title and select the "Create AAC Version" option from the context menu. Allow several seconds for iTunes to create a new version of the song.
Click the new 30 second AAC version of the song and drag it onto the computer's desktop. After the file has copied to the desktop, delete the file from iTunes. Right-click the song file and select the "Get Info" option. Click the "Name and Extension" subheading and change the file's extension from ".m4a" to ".m4r."
Close the "Get Info" window and double-click on the new ".m4r" file to add it to iTunes. Plug your iPhone into the computer and sync it with iTunes.
Tap the "Settings" icon on the iPhone's main menu, followed by the "Sounds" icon. Tap the "Ringtones" icon and select the ringtone you added to your iPhone to set it as your ringtone.