How to make a blog tab in every user profile
Using a custom module, you can implement hook_menu() in the following way:

function mymodule_menu() {
  $items['user/%user/blog'] = array(
    'title' =&gt; 'My blog', 
    'page callback' =&gt; 'blog_page_user', 
    'page arguments' =&gt; array(1), 
    'access callback' =&gt; 'blog_page_user_access', 
    'access arguments' =&gt; array(1),
    'file path' =&gt; drupal_get_path('module', 'blog'), 
    'file' =&gt; 'blog.pages.inc',
  );

  return $items;
}


The "file path" property is used because the module that defines the menu is not the one containing the "blog.pages.inc" file.

If you have few experience on how to create a Drupal module, you should start looking at:


Creating Our First Module using Drupal 6 (Part1)
Creating a Simple Drupal Module
Creating Drupal 6.x modules
Creating Drupal 7.x modules