Centering a resposive Site a beginner question

Hey,

I´m developing a site with joomla 3.2 and t3

I have already started but I got stuck. This is what I got http://stl1.bplaced.net/

My problem is I want the whole Site to be centered like this site http://www.tabtech.de/ an behave responisve like this site.

Right now I cheat by centering the header picture and putting Spotlights under it. Then I left one box empty and started with the second one so that it seems like I centered the site.

Is there a better or easier solution because ists no a good one.

I think ists really complicated to explain my Problem...

Thank you for your answers!