Hi,
This is a skinning question, not about site administration. To learn how to do it I recommend you study the skin andreasviklund-02 which has 2 levels of horizontal menu and andreasviklund-02-alt1 which has 3 levels of horizontal menu. Look especially at how the menus are configured in the layout.master file.
All I did on this site is take the styleshout-brightsideoflife skin and apply the same techniques from andreasviklund-02 to make the second level tabs on this site. I also made it wider which required some tricky business adjusting image sizes. Any pages deeper than 2 levels on this site use a vertical menu on the left.
Hope it helps,
Joe