For a fresh install of Raspberry Pi OS Lite with no desktop place your micro SD card in the Pi connect your monitor, keyboard, and mouse. DO NOT connect your network cable. (This is for security reasons. The Pi is configured with a default username and password.)  Then connect your power cord. Connecting your power cord first will power up the pi but when you go to connect other devices like a keyboard and mouse you might cause it to do a hard reboot possibly damaging your microSD card. Once the pi is powered up you should see a login prompt. Like this.

Raspbian GNU/Linux 11 raspberrypi tty1

raspberrypi login: _

The default username is: 

pi

The default password is: 

raspberry

NOW THAT YOU ARE LOGGED IN BEFORE YOU DO ANYTHING ELSE!!!

You need to change your password. You do this very simply by typing passwd entering in the current/or default password than entering twice the new password. Make sure you do this as it won't take long for malware to find your new device and login and add malware and back doors. 

passwd

Here are some other common default logons. 

Raspberry Pi Distributions Username Password
Raspberry Pi OS pi raspberry
Kali Linux root toor
OpenELEC root openelec
Arch Linux ARM root root
Debian pi raspberry
LibreELEC root libreelec
OSMC osmc osmc
QtonPi root rootme
Ubuntu Server ubuntu ubuntu
Retropie pi raspberry