/etc/network/interfaces

auto wlan0
iface wlan0 inet dhcp
wpa-conf /etc/wpa_supplicant.conf

/etc/wpa_supplicant.conf

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