By default when you first install Raspberry PI OS the user is called PI and the password is set to raspberry.
You should change the password at the earliest opportunity as it is insecure to leave it as the default one.
Fortunately changing the password is a straight forward procedure and is outlined in this article.
Click on the menu icon in the top left corner, navigate down to preferences and choose “Raspberry PI Configuration”.
The Raspberry PI configuration screen will load and there are loads of settings that you can amend via this screen.
For the purposes of this article simply click on “Change Password”.
Enter the password you wish to associate with the PI user and click “OK”.,
Your password will now be amended.