HELP NEEDED TO MAKE TEST CASE

I had to make a project for that i choose a the topic " AES encryptionin using C language " I have to make the test case but i don´t know how to make a test case . i have attached a file it includes example of stepwise execution of AES algo .The AES algo is given in this link <a href="https://en.wikipedia.org/wiki/Advanced_Encryption_Standard" target="_blank" rel="nofollow noopener noreferrer">Advanced Encryption Standard</a>
Can someone guide me how to write a test case though i searched the internet but was not able to cleary understand it.
Please can anyone guide me through how to write it .