How do I overwrite the page? I used the CCK module to create a content type that has three fields (title, description, and body). I want to overwrite two pages: one is where there is a "create content" navigation located on the left page; the other one is where I click  on "Create page" and which displays the form page.
To alter a node edit from, you need to create a custom module that implements hook_form_alter(), using code similar to the following one:

function mymodule_form_alter(&amp;$form, $form_state, $form_id) {
  if (isset($form['type']) &amp;&amp; isset($form['#node']) &amp;&amp; $form['type']['#value'] . '_node_form' == $form_id) {
    $node = $form['#node'];
    // ...
    // @todo Add the code you need.
    // ...
  }
}


For examples of actual implementations of hook_form_alter() that alter the node edit form, see book_form_alter(), locale_form_alter() (which alters also other forms), menu_form_alter(), path_form_alter(), and taxonomy_form_alter().