what is starting point of any drupal module.?
what is the first function that executes in .module file? - none in the C sense. If you came from C, C++ or C# environment, think of modules like you think of dll / so files. If you came from Java, think of them like you think of libraries. Piece of code that is hooked by the program if needed, and provides functions that can be called as needed.

So which function is called first? One that implements hook that, by accident, happens to be needed first.

Drupal modules provide functions that react to certain events. These functions are called hooks. Introduction to hooks is in the Develop for Drupal documentation.

Note: As  Ayesh K helpfully pointed, PHP code placed in module file will usually be executed at every call. But Drupal expects there is no code outside functions in .module files. Placing direct PHP code may cause unexpected behaviour, so please don't do it.