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Even the digital property liberators/internet pirates don’t test their software. I feel like I’m on an island with a small population of test enthusiasts.
You start with listing requirements (what a program/a class/a function is expected to do or not to do) and then write tests that verify that it is indeed so.
The easiest thing is writing unit tests. Pick a function, define requiremens and write a test for every requirement. If your code is not very modular and it is difficult to isolate a class or a function in order to test it, then maybe you should refactor the code first.