You should always post a link to your site so someone can inspect it. You should be able to do this with some CSS that changes the display of all of the menu items to block, but as I mentioned, it would be easier to offer more specific advice if we could see the site.
Thanks for your answer! Here is the website http://rodrigoaffonso.com
OK, you should a CSS plugin like Custom CSS Manager instead of editing the theme’s files directly. Or you can create a child theme, but that’s kind of a pain that you don’t need to go through if you’re just making a CSS change.
Adding this rule should expand all of your sub-menu items:
#main_menu ul {
display: block !important;
}