1) Download Nice Menus Module from drupal website
2) Extract it into /sites/all/modules
3) Log in as Admin & Enable it from Module section
4) Open page.tpl.php from your theme folder & find menu section code than replace with below code:-
// id, direction, depth should have the values you want them to have.
$menu = theme('nice_menus', array('id' => 0, 'direction' => 'down', 'depth' => 1, 'menu_name' => 'main-menu', 'menu' => NULL)); print $menu['content'];