Step 1) Ignore the script part of this assignment. How do you create a user, create its home directory, set its password, and lock it on the command line? Once you know what commands you need, stick them in a file and call it a script. That's all a script is, a sequence of standard commands you type on the command line. Once you know the commands, creating a script is nothing more than copying and pasting them into a text file.
Step 2) Add input parsing. How do you read a file to get the user names? Add it to your script and wrap your above commands inside a loop over these user names.
Step 3) Add error checking. How do you check if a user already exists? Add it to your script.
If you run into a problem, ask, but it needs to be a specific problem, not the entire assignment verbatim.