Make test class work only on one class

Hello, I have a question. When I make a test class to a test a certain class and I have other classes in that same file, and lets say these other classes have some error in them. In the test Class, I only want to reference the class I want to test but my program doesn't work because it sees the other classes which I'm not testing have errors. How do I make sure that other classes which I'm not testing will not affect the  test class..