How to Setup Wi-Fi On Your Raspberry Pi via the Command Line

Configure Your AP Details

sudo nano /etc/wpa_supplicant/wpa_supplicant.conf

Make sure the file reads like this

ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
update_config=1

network={
    ssid="SSID"
    proto=RSN
    key_mgmt=WPA-PSK
    pairwise=CCMP TKIP
    group=CCMP TKIP
    psk="yourkey"
}

Configuring the interface

 sudo nano /etc/network/interfaces

Add the following

allow-hotplug wlan0
iface wlan0 inet manual
wpa-roam /etc/wpa_supplicant/wpa_supplicant.conf
iface default inet dhcp

sudo ifconfig wlan0 down
sudo ifconfig wlan0 up

Reboot. Done.

Model B