By default, menus in Joomla 3.0 goes vertically. It's not a problem to make it horizontal.
To make the menu horizonal, you can simply fix Menu Class Suffix.
Go to modules, open the menu module that you've created. Once the menu module is open, click on the "Options" tab and then click on "Advanced Options". In the "Menu Class Suffix" field, type in " nav-pills". Please be sure to hit the space bar one time before typing "nav-pills" to successfully apply the menu class suffix. Save your work.
Refresh page at front end and you will find miracle changes in menu style!
Screenshot below can help you to find settings.