Audacity Tutorial 2
Audacity is free software for Mac or PC, it doesn’t have all the functionality of Garage Band but the end result is just as good. Download it from http://audacity.sourceforge.net/download/. Open it up on your desktop, click the red circle to record and start talking, click the yellow square when you have finished. You can improve the quality massively by using a microphone. (Any will do, you can even plug headphones into your audio-in socket and talk into the ear-piece.) Highlight the areas you want to delete and click backspace on your keyboard to remove them.
Use Track>Add New> to add a new voice or background music, remember to mute everything else while you record. You can find lots of links to Creative Commons and license free music here
When you have finished click Export and choose the file type you want to save as. If you choose WAV then iTunes will be able to play the file. Some players need your file to be in MP3 format, you may need to download a LAME encoder to do this, try saving as an mp3 and follow the on screen instructions.