I have a iPhone 6 Plus and was wondering if I can use a song that I purchased in iTunes as a ringtone for my iPhone?
Yes. Follow this guide or any other after getting the song from ~/Music/iTunes/iTunes Music.
Select and edit a song
The first thing we'll need to do is choose a song in iTunes to use as a ringtone. Open up iTunes on your Mac or PC and click on the My Music tab. Choose a song that's on your hard drive then right click on it, choose Get Info, then click the Options tab.
Within the main panel you'll see settings for Start and Stop. A ringtone can only be thirty seconds long, so use these settings to isolate the part of the song you want then click OK. You won't be deleting anything on the actual track itself, so don't worry.
Now highlight the song, go up to File at the top of the screen and select Create New Version>Create AAC version. iTunes will duplicate the track, but if you look closely you'll see that the new version is only thirty seconds long. That's the one we'll use for our ringtone. Remember to go back to the original track now and untick the Start and Stop options, otherwise the track will only play that short section.
Change the file type
Highlight the short version of the song, then right click and select Show in Finder. Now you'll see the two versions of the song. The duplicate should have a 1 at the end of the name to differentiate it, and of course the file size will be smaller. To use it as a ringtone we'll need to convert the file type from .m4a to .m4r, which involves clicking on the name once to highlight it, then once again to edit the name so we can change the last three letters. Finder will then ask you if you are sure you want to use the new file type? Choose 'Use .m4r' in the pop up box to confirm. Leave the Finder window open for now, as we'll use it again in a moment.
Delete the shorter track
Return to iTunes and delete the short version of the track, remembering to check the length carefully before you do so. When prompted choose Keep File, as we want to remove the track from our music library but not our computer entirely.
Import the track to iTunes as a ringtone
Go back to the Finder window and double click the shorter file. Rather disconcertingly it will disappear from the box, but start playing in the background. Don't worry, it isn't really gone. Return to iTunes, go to the row of icons in the top left corner and click on the three dots. This brings up more options, one of which is Tones. Click this and you'll see that the short version of the song is now a ringtone.
Sync the ringtone with your iPhone
Connect your iPhone to iTunes and click on the little phone icon just to the right of the three dots. This will bring up the iTunes panel for your phone, from which you want to select Tones in the left hand column. Once inside click Sync Tones>Selected tones>then the track you've created. Finally go to the bottom right hand corner and click Apply.
Set the new ringtone
All that remains to be done now is to go to Settings on your iPhone, then Sounds, and you'll find the new ringtone at the top of the list. Tap on it to select it as your general tone.