How to I write code to test code from another class?

So I'm supposed to write code to test a method from a test class however I'm totally lost. I guess I'm just confused whether I have to right regular code such as for loops or if I'm able to do it another way. I'm new to java and it's causing me a ton of trouble!

Here is one such method from the interface I have to test:

    // Returns a reversed version of the specified string,
    // which can contain any ASCII character.
    // For example, returns "edcba" for "abcde" 
    public String reverseString(String s);