Writing a Drupal module involves several steps, including:

1. Planning: Determine the purpose of the module and what it should do.

2. Creating the module directory: Create a directory for your module in the "modules" directory of your Drupal installation. The name of the directory should match the name of the module.

3. Creating the .info file: Create a .info file in the module directory with information about the module, including its name, description, version number, and dependencies.

4. Creating the .module file: Create a .module file in the module directory. This file will contain the implementation of the module's functionality.

5. Defining hooks: Implement Drupal hooks, which allow your module to interact with the core functionality of Drupal and other modules.

6. Creating database tables: If necessary, create database tables to store data used by the module.

7. Implementing the module's functionality: Write the code that implements the module's functionality.

8. Testing: Test the module to ensure it works as expected.

9. Documentation: Document the module, including its purpose, installation instructions, and usage instructions.

Here is a simple example of a Drupal module that adds a greeting to the page:

<?php

/**
 * Implements hook_block_info().
 */
function simple_greeting_block_info() {
  $blocks['simple_greeting'] = array(
    'info' => t('Simple Greeting'),
    'status' => TRUE,
  );
  return $blocks;
}

/**
 * Implements hook_block_view().
 */
function simple_greeting_block_view($delta = '') {
  $block = array();
  switch ($delta) {
    case 'simple_greeting':
      $block['subject'] = t('Greeting');
      $block['content'] = t('Hello, world!');
      break;
  }
  return $block;
}

This is a basic example and there is much more you can do with Drupal modules, including adding forms, using the database, and integrating with other parts of Drupal. I recommend looking at the official Drupal documentation and the examples of existing Drupal modules for more information.