Software testing is a important part of software development.
In earlier days testing of a software was typically done at the end of the development cycle. Since this is very late to uncover severe problems, today's approach relies heavily on test automation. An important part for this is testing individual pieces of the code (unit testing). Even more, with Test-Driven-Development (TDD) tests are written before implementing a part of the application.
Many actual programming languages provide appropriate tools for automated test execution. The VBA programming language and the Code Editor (the VBE - Visual Basic Editor) unfortunately only contain rudimentary functionality to support testing of code (Debug.Print, Debug.Assert).
The following pages will help you to write the first tests for your application.