Is there any online tutorial explains how can I map the requested URL to actions' code in drupal 7? I'm beginner to Drupal 7. As I know, when the user request for example the URL: (www.example.com/node/123) then the Node module will responsible on responding to this request. How can I create a simple php function in my custom module so that it can respond to a specific request generated from my custom page or my custom form submission? for example: www.example.com/mymodule/blablabla?
You are looking for hook_menu(), you will need to implement this hook on your custom module to register path in drupal.


  Define menu items and page callbacks.
  
  This hook enables modules to register paths in order to define how URL
  requests are handled. Paths may be registered for URL handling only,
  or they can register a link to be placed in a menu (usually the
  Navigation menu). A path and its associated information is commonly
  called a "menu router item". This hook is rarely called (for example,
  when modules are enabled), and its results are cached in the database.


Since you are beginner, First I would suggest you to learn about hook system for drupal.

Checkout this answer for detailed use of hook_menu.