Equivalence Class Testing

I have an assignment at school and I have been asked to create an equivalence class testing for a given case. Tha case is:

One of the programmers in your group was asked to write a program that has two strings of alphanumeric characters (possibly with spaces, but no special characters) to enter and check to see if one string is a true substring of the other. Real substrings are those that are not trivial. For example, hello is not a true substring of hello because it's the same string. Similarly, the empty string is not a true substring of another string. Develop an equivalence class test scheme for testing this function. How many tests do you have in the end? Are any of these tests exclusive to the acceptance test and can not be applied to functional testing?

I know what equivalence testing but only with simple examples, i.e. if age is between 18 and 60. This case is confusing me. How do I initiate a test, for example, to test substrings? And what would be my boundary values in this case?