Steps for Raspberry Pi 3

Install the Latest Patches

$ sudo apt-get update
$sudo apt-get upgrade

Scan for ESSIDs

$ sudo iwlist wlan0 scan | grep SSID

Save a Copy wpa_supplicant.conf File

$ cd /etc/wpa_supplicant

$ sudo cp wpa_supplicant.conf wpa_supplicant.conf_original

Modify wpa_supplicant.conf File

$ sudo nano /etc/wpa_supplicant/wpa_supplicant.conf

Add the following code

network={
ssid="SSID"
psk="WIFI PASSWORD"
}

Stop/Start Wlan0

sudo ifdown wlan0

sudo ifup wlan0