how to make a ringtone for your iPhone using iTunes
this will only work on unprotected files, scroll to the bottom of the post for a quick explanation of how to determine whether your song is protected
1. open iTunes and make sure your settings are set to create a ringtone (steps 2-3).
2. go to "Preferences" (i think this is under "File" on a pc), select "Import Settings" under the "General" tab
3. Select "AAC Encoder" under "Import Using:"
4. pick the song you want to create into a ringtone. listen to the song and decide what part of it you want to use as the ringtone - you have up to 40 seconds
5. ctrl+click (right click on a pc) the song and select "get info"
6. select the "options" tab, put in the desired start and stop time (again, up to :40), and press ok
7. ctrl+click your song and select "Create AAC Version". this will create a new file of the song with your start and stop time. 
8. drag the new file (which will be shorter than the original) to the desktop
9. after dragging it to the desktop, delete the new file from iTunes. you must do this or you iTunes will not import the ringtone. at some point you will want to go back and deselect the start and stop time on the original file so your song will play normally
9. rename the file. change the extension from ."m4a" to ".m4r". if prompted, select "Use .m4r"
10. drag the new .m4r file into your iTunes library. if you have done everything correctly, it should show up in the ringtones playlist
11. sync your iPhone or drag the ringtone into your iPhone if you manage it manually
12. you're done
how to recognize file types:
this only works on unprotected files. most songs that you buy on iTunes now are no longer protected. you can see what type of file it is by ctrl+clicking (right click on a pc) the toolbar where it says Song, Artist, Album etc. and selecting "Kind"
if the file is a "Protected AAC" it will not work, but if it is a "Purchased AAC" or "MPEG Audio File" or "AAC" it will work.
there is a way to "unprotect" protected files, but it's a bit of a pain
