This is by far the the fastest, most simple way to crack wifi, the whole thing is a walkthrough, so read on and lets crack that WiFi!
1. Install first python since the script is written in python language.

sudo apt-get install python-tk

2. Install aircrack-ng suite:

sudo apt-get install aircrack-ng

3. Install mac changer:

sudo apt-get install macchanger

4. Execute wifite script and you can download here:

wget http://wifite.googlecode.com/svn/trunk/wifite.py 

5. Change the permission to be executed:

chmod +x wifite.py 

6. and run/execute the script:

python wifite.py

Need more help type

python wifite.py

Happy WiFi hacking! 😀
UPDATE; This does work for both WEP & WPA but with WPA your going to need some word files or rainbow tables to break the handshake.