Unlike the old days, creating a custom ringtone of your favourite song is easy peasy. You don't even need to use your PC!
1) Assume you already have your song in your Android phone, run Ringdroid and select your song.
2) The next screen might seem complicated, but it is as simple as it gets. (See screenshot below) What's important here are the two sliders, one on the top left and one on the bottom right.
Top left - drag to choose the start point
Bottom right - drag to choose the end point
3) Press the play button to preview your ringtone, when you're happy press the Diskette (save) icon.
Enjoy your new ringtone :)