How do I show user attributes on articles?
If you take a look at node.tpl.php, you'll notice there's already a variable for this type of data: $submitted. You could replace that variable with your own data, but to do it the Drupal way, what you want to do is modify the value of $submitted using a preprocess function.

In your theme, create a template.php file (if you don't have one already). In it, add the following function (replacing example with the short name of your theme):

function example_preprocess_node(&amp;$variables) {
  // Linkify the URL
  $twitter_url = l('http://twitter.com/someusername', 'http://twitter.com/someusername');

  // This is the same thing as saying 
  // $variables['submitted'] = $variables['submitted'] . ' ' . $twitter_url;
  $variables['submitted'] .= ' ' . $twitter_url;
}


Clear the cache (Configuration &rarr; Performance &rarr; Clear cache), and check your node pages again: you should see the Twitter URL appended to submitted line.

But this is of course, static: to load the user's Twitter URL, you'll need to load the field data:

function example_preprocess_node(&amp;$variables) {
  // Even though there's a user object in $variables['user'], we need to
  // load a new copy because the preloaded one doesn't have have field data
  $user = user_load($variables['uid']);

  // Retrieve the data and its value for the Twitter URL field
  // Replace field_twitter_url with the short name of the field
  // found at admin/config/people/accounts/fields
  $items = field_get_items('user', $user, 'field_twitter_url');
  $field = field_view_value('user', $user, 'field_twitter_url', $items[0]);

  // Get the rendered output of the field for use in the submitted line
  $twitter_url = drupal_render($field));

  // If you wanted to get more fields, repeat the last three lines, replacing
  // field_twitter_url with the names of the other fields.

  // Linkify the URL and append it to the $submitted variable
  $variables['submitted'] .= ' ' . l($twitter_url, $twitter_url);
}


The Field API isn't the most intuitive or straightforward API in the world, but here's what's going on:


Load the user object, which contains the information about the fields attached to it.
Load the specific data associated with the field_twitter_url using field_get_items().
Get the render array for the specific field you want to display using field_view_value().
Get the final rendered output using drupal_render().


Now, if you wanted to have the Twitter URL separate (so you can move it to where ever you want in the template), just create your own variable:

function example_preprocess_node(&amp;$variables) {
   // ...
   // Do all the stuff from the previous sample function, except
   // setting $variables['submitted']
   // ...

   $variables['twitter_url'] = $twitter_url;
}


Now, you can add $twitter_url to anywhere in node.tpl.php. If your theme doesn't already have its own node.tpl.php, navigate to the modules/node folder, copy the node.tpl.php that's there to your theme folder, and clear the cache.