Beginners should definitely read the aricrack documentation from end-to-end, it's here:
http://www.wirelessdefence.org/Conte...kORIGINAL.html
The link on wirelessdefense also gives step-by-step instructions for using the aircrack suite and examples:
http://www.wirelessdefence.org/Conte...rcrackMain.htm
Read that little lot, *then* ask more detailed and specific questions here :-)