Use the world's lightest testing tool.
Starting with unit tests is a bit difficult—you just don't know what to do. Here's some free software to give you a leg up.
Kent Beck wrote a testing framework, called SUnit, for Smalltalk some years ago. It caught on all over, especially in XP teams using Smalltalk. He and Erich Gamma then provided the JUnit tool for Java. Today there are CppUnit, PerlUnit, PyUnit for Python, VBUnit for Visual Basic, and many other languages as well. These frameworks are available on Ron Jeffries' web site, http://www.XProgramming.com. Click the "software" link.
Within the limitations of the language being tested, all the frameworks work the same ...