Are you making it for your iPhone, if you have one? If so, its really easy. What i will tell you works for Windows. I dont know the Mac way.
First Right Click the song and click Options on the top
Then you will see a two boxes with the words start time and stop time next to them
Check both of them and choose an oppropriate time for the ringtone.
After that click ok, and play the song to make sure it fits your needs. If it doesnt, go back and do it again.
After that, right click the song and click conver to AAC version... This will give you a second copy of the song.
Right click the AAC version and it click open in Windows Explorer.
From their make sure you have "Show Hidden Extensions" unchecked and change the .m4a ending to .m4r.
Double click the song and it will add itself as a ringtone on itunes
Used this way for my iPhone. Works a lot cleaner than audio editing software