There is a plethora of information not only on this forum but throughout the web on how to do this.
I would suggest starting at http://www.aircrack-ng.org/doku.php?id=cracking_wpa and then use what ever method you feel comfortable with to crack the .cap (there are also online services for this but to date I have not had any success with them)
If still having difficulties use Google. (Google can be you're best friend)
Rab.