Drupal - 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']);
}