What is a Test Organization?
A test organization illustrates who is responsible for the type of activity in the test process. Such that every organization defines the test functions, test facilities and test activities. Test Organization defines the competencies and knowledge of the people involved.
In the process of testing, generally everyone is considered responsible for quality in one or the other form. The structure of testing must replicate the associated system development approach, be it Agile, waterfall or a hybrid form. In many cases. Therefore it translates to a project or team organization with testing either as an integral part of the project/team or as a separate, possibly independent, entity. Also it is suggested to have frequent testing, especially in combination with scarce test knowledge and experience, testing can also be organized in a line or staff function, the so called permanent test organization.
Typical, unit tests are created in a separate project or separate source folder to keep the test code separate from the real code. Now, what should be tested is a highly controversial topic. Some developers believe every statement in your code should be tested.
So as you start developing tests for an existing code base without any tests, it is good practice to start writing tests for the parts of the application in which most of the errors happened in the past. This way you can focus on the critical parts of your application.