How to test your program properly? [closed]

I'm trying to learn how to test my program properly, and I've got a question: When we want to test our program,I know we should use test cases and see if they act as expected, but how can we count on couple of tests? There is wide range of possibilities. Is there a tool like a method/class or something else that is able to test a range of variables and see if they act as expected ? How do you test your program?

Thank You