interfaces

allow-hotplug wlan0
auto wlan0
iface wlan0 inet static
    address 192.168.2.4
    netmask 255.255.255.0
    gateway 192.168.2.1
    essid "your_essid_here"
    wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf

wpa_supplicant.conf

country=YOUR_COUNTRY_CODE
ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
update_config=1
network={
        #key_mgmt=WPA-PSK
        ssid="your_essid_here"
        #proto=RSN
        #pairwise=CCMP
        #auth_alg=OPEN
        psk="your_password_here"
}