Audacity..
http://audacity.sourceforge.net/
Lame if needed to encode to mp3:
http://lame.sourceforge.net/
How to install Lame:
http://audacity.sourceforge.net/help/faq?s=install&i=lame-mp3
Basic tutorials:
http://audacity.sourceforge.net/manual-1.2/tutorials.html
http://www.recipester.org/Software/Audacity
Select section to keep.
Go to Edit and select "trim"
If start of the track is selected no further action is needed.
If a section mid song is selected it will need to be moved to the start of the track using the time shift tool.
(the icon with the left/right arrow). Click, hold and drag on the section of waveform and move it to the left.
Export and save via the file menu to the format of your choice.