Depends if you want to do it from your computer or phone.
I use Audacity on my PC, and snip the section of a song I want as a ringtone, then export it as MP3, and transfer it over onto my phone via bluetooth/dropbox/USB mount/etc.
There are also a few "ringtone editors/makers" on the Playstore that you can play around with that basically do the same thing, although I find using a computer is easier and more accurate in terms of start/stop times.
Lastly, I believe there are apps that let you download ringtones directly (some paid, some free). That would be the simplest way if you don't care about song selection.