Mount the main partition of memory card on your computer.
$ sudo mount /dev/sdb2 /mnt
Now you need to edit wpa_supplicant.conf to include your wifi information.
$ vim /mnt/etc/wpa_supplicant/wpa_supplicant.conf
country=GB
ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
update_config=1
# Add this block
network={
ssid="ENTER-SSID-HERE"
psk="ENTER-PASSWORD-HERE"
}