Must new tables be created in hook_update_N()?
So just a copy paste from drupal.org. You also need to add the schema definition to hook_schema.

/**
 * Create new database table {mytable2}.
 */
function mymodule_update_7101() {
  $schema['mytable2'] = array(
     // table definition array goes here
  );
  db_create_table('mytable2', $schema['mytable2']);
}