At first, you have make some changes to /etc/apt/sources.list. You have to add following line to every line which starts with deb (in /etc/apt/sources).

contrib non-free
Then, you have to update your system.

sudo apt-get update && apt-get upgrade
Then, install firmware-iwlwifi. (Your default user won't be sudoers, that's why you have to work as a root user su)

apt-get install firmware-iwlwifi
Then, you will have to reboot your PC.

wpa_passphrase ssid passwd | tee -a /etc/wpa_supplicant/wpa_supplicant.conf