Connect to a headless Raspberry Pi through SSH for the first time

This will work for Raspberry Pi OS (formerly known as Raspbian) and no monitor or keyboard are needed.

After flashing the OS, create a wpa_supplicant.conf file on the boot partition of your SD Card (not the boot folder).

ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
update_config=1
country=<Insert 2 letter ISO 3166-1 country code here>

network={
 ssid="<Name of your wireless LAN>"
 psk="<Password for your wireless LAN>"
}

On the same partition, create an empty file called ssh. It will instruct the OS to enable the SSH server. Also, create your SSH user by creating a file named userconf.txt.

touch ssh

echo <user>:`echo '<pass>' | openssl passwd -6 -stdin` > userconf.txt

To find the Raspberry Pi in your local network, you can use nmap. Assuming your local addresses start with 192.168.0, run:

nmap -sn 192.168.0.0/24

If your Pi connected correctly, you will see something similar to the following in the output.

Nmap scan report for raspberrypi (<IP>)
Host is up (0.11s latency).
MAC Address: <MAC> (Raspberry Pi Trading)

Further reading

Official docs on remote access and Wi-Fi settings.

Security concerns on default user and password for SSH