Setting up wireless internet on Raspberry Pi

Confirm the compatibility of your device here.
https://help.ubuntu.com/community/WifiDocs/WirelessCardsSupported

This guide assumes using Raspbian release 2014-12-24.

If you have not yet installed wpa_supplicant, install it by running the following command.
sudo apt-get install wpasupplicant

Add the following lines to the end of /etc/wpa_supplicant/wpa_supplicant.conf.
network={
    ssid="SSID"
    scan_ssid=1
    psk="KEY"
}

Restart your wireless network interface by running the following commands.

sudo ifdown wlan0

sudo ifup wlan0

Good luck.

Reference:

https://help.ubuntu.com/community/WifiDocs/WirelessNetworkOfUniversityOfMaastricht