Linux Script--. Arguement checking user login

The directions: (basic IF ELSE ELIF or CASE)

Ask the user's login Test to see if an argument was provided If not, display a message and end the script Check to see if the user is in the system in the /etc/passwd file. If so, the script will print Found User {user} on our system. Otherwise, it will print No such user on our system.

I'm horrible at Linux Script, needing some guidance. Lol