You've not stated what software you've got available. This is by far the most popular free sound editor:
http://audacity.sourceforge.net/
You should note that, for patent reasons, Audacity in its native form can't save files in MP3 format. To get it do so so you have to follow these instructions:
http://audacity.sourc...?s=install&i=lame-mp3
(That's just a 'one-off' process. Once you've done it you'll always be able to choose 'MP3' as your file format).
Audacity is a fairly intuitive program (and it's really easy to do what you want to do) but it's still a lot easier to use if you read the documentation first!
http://audacity.sourc...et/help/documentation
(Yes, I admit it. I'm a typical bloke. I spent ages trying to work out how to do certain things with Audacity before reading the instructions and discovering how easy the tasks really were!).
Chris