I'd like to know how to make test cases when input are strings

I'm studying S/W testing and practicing to make test case. But It is so ambiguous to me, when inputs are strings. I have a example. please give me some advise how to partition classes, and determine boundaries.

My program has to inputs that are 2 non empty Strings. And It determines which one has more vowels characters and prints the string.

For example, when 'mike' and 'hi' is entered, the program prints mike. If both strings have same number of vowels, the program prints equal.