Creating custom field with drop down on new post page

Instead of defining the default values for custom fields, you should do like Milo says.
You should have something like this in your functions.php.
I tried to follow what you need, but didn’t test it.

<?php

// action to add meta boxes
add_action( 'add_meta_boxes', 'voodoo_dropdown_metabox' );
// action on saving post
add_action( 'save_post', 'voodoo_dropdown_save' );

// function that creates the new metabox that will show on post
function voodoo_dropdown_metabox() {
    add_meta_box( 
        'voodoo_dropdown',  // unique id
        __( 'Voodoo Dropdown', 'mytheme_textdomain' ),  // metabox title
        'voodoo_dropdown_display',  // callback to show the dropdown
        'post'   // post type
    );
}

// voodoo dropdown display
function voodoo_dropdown_display( $post ) {

  // Use nonce for verification
  wp_nonce_field( basename( __FILE__ ), 'voodoo_dropdown_nonce' );

  // get current value
  $dropdown_value = get_post_meta( get_the_ID(), 'voodoo_dropdown', true );
  ?>
    <select name="voodoo_dropdown" id="voodoo_dropdown">
        <option value="USA" <?php if($dropdown_value == 'USA') echo 'selected'; ?>>USA</option>
        <option value="Canada" <?php if($dropdown_value == 'Canada') echo 'selected'; ?>>Canada</option>
        <option value="Mexico" <?php if($dropdown_value == 'Mexico') echo 'selected'; ?>>MEXICO</option>
    </select>
  <?php
}

// dropdown saving
function voodoo_dropdown_save( $post_id ) {

    // if doing autosave don't do nothing
  if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) 
      return;

  // verify nonce
  if ( !wp_verify_nonce( $_POST['voodoo_dropdown_nonce'], basename( __FILE__ ) ) )
      return;


  // Check permissions
  if ( 'page' == $_POST['post_type'] ) 
  {
    if ( !current_user_can( 'edit_page', $post_id ) )
        return;
  }
  else
  {
    if ( !current_user_can( 'edit_post', $post_id ) )
        return;
  }

  // save the new value of the dropdown
  $new_value = $_POST['voodoo_dropdown'];
  update_post_meta( $post_id, 'voodoo_dropdown', $new_value );
}
?>

If you think this is too much complicated you can use a metaboxes plugin, find one in WordPress Plugin directory that has already dropdowns.