To configure the wireless connection of a newly installed Pi or an old one with an obsolete wireless config, just create a file wpa_supplicant.conf file on the /boot/ fat32 partition with this content:
ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
country=GB
update_config=1
network={
ssid="<Name of your wireless LAN>"
psk="<Password for your wireless LAN>"
}
Start it and wait.