How can hack a wifi password??

Why would you want to :)

As a white hat working in a computer security company we use various penetration tools. For this purpose you could use Reaver to exploit WPS vulnerability or air-crack-ng . You could try out these penetration tools in a nice Linux package as Backtrack Linux or Kali Linux.

Please hack only your own network or equipment. Accessing other network, equipment without permission is illegal.

use kali linux.

you can do it using aerodump and aireply tools normally used by default in Linux Backtrack, but you can install them in other distributions.