iTunes is a popular choice to convert MP3 to iPhone ringtone. It's available on both Mac and Windows. Although it's a bit complicated for new users, once you go through the whole process, you will find that it's quite easy to handle. If you don't have iTunes on your computer, you can download the compatible version on the Apple official website: https://www.apple.com/itunes/
Use MP3 as iPhone Ringtone
Step 1. Open iTunes. Add the MP3 song to the library by clicking on File > Add File to Library.
Step 2. Right click on the MP3 audio and select Song Info.
Step 3. Go to the Options tab and set the start time and the stop time of the ringtone. Please note that Apple limits all ringtones to 40 seconds. After you are finished with the settings, click on OK.
Convert MP3 to AAC
Step 4. Go to File on the upper-left corner > Convert > Create AAC Version. If you don't find the option, you can go to the Edit menu > Preferences. The General Preferences window will pop up. Just click on the Import Settings button, select AAC encoder from the Import Using drop-down list and click on OK.
Step 5. Change the extension of the converted audio file from .m4a to .m4r.
Tips. To find the output folder for saving the converted audio file, go to the Edit menu > Preferences, and select the Advanced tab. You can also change the output folder there.
Step 6. Now connect your iPhone to iTunes and drag and drop the M4R audio file to the Tones tab under iPhone. After that, you can set the audio as your iPhone ringtone.
Also Read: Convert APE to iTunes l Convert YouTube to iTunes