css for my homepage only

I need to know what file and were to go to change the css for my homepage to my site.

I would like to change the location of the navigation bar and make it smaller. I would like to move the nav. bar just below the promo  but smaller in size and with no frame.

Can some one please explain what to do 

thanks I need it

Use firebug firefox extension:  https://addons.mozilla.org/en-us/firefox/addon/firebug/

Use firebug in Firefox.