Make new file /boot/wpa_supplicant.conf:
ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
update_config=1
country="{Your countries' ISO 3166-1 code}"
network={
ssid="{Name of Network}"
psk="{Password}"
}
raspberry pi wifi headless
DONT FORGET TO ADD AN EMPTY ssh FILE TO THE BOOT!!