sudo nano /etc/network/interfaces

auto wlan0

iface lo inet loopback
iface eth0 inet dhcp

allow-hotplug wlan0
iface wlan0 inet dhcp
wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf
iface default inet dhcp
sudo nano /etc/wpa_supplicant/wpa_supplicant.conf

network={
	ssid="SSID"
	scan_ssid=1
	psk="secret"
	proto=RSN
	key_mgmt=WPA-PSK
	pairwise=CCMP
	auth_alg=OPEN
}