Install reaver from here
Install air crack tools here
Or install it from standard repo, in ubuntu like
apt-get install aircrack-ng
for the next type
find your wireless interface, in ubuntu like wlan0.
ifconfig wlan0 up
Next command airmon-ng start wlan0. It will make new interface mon0 for monitor mode.
airodump-ng -i mon0
View all around wifi networks and choose one to hack. Remember channel and mac address.
Then wait for program will find needed wpa key with pin.
It will be bruted not longer than 20 hours.