If you are going to use Imbalance 2 then I'll save both you and I some time. (Note what I post below is not about CSS editing at all).
Imbalance 2 offers two layout options, fluid and fixed.
Fluid means the width changes according to screen resolution (or the width of browser) your visitors use.
Fixed means the width does not change according to screen resolution (or the width of browser) your visitors use.
Column one is the blog title and tagline; you edit them in Settings > General.
Columns two and three are for two custom menus; you create and load them in Appearance > Menus - instructions here: http://en.support.wordpress.com/menus/
Column four is for widgets; you add them in Appearance > Widgets.
You need to set a featured image in each post in order to see the thumbnail in imbalance 2. Click on the module that has 'featured image' in it (if you don't see the module, look at the top of the editor window for 'screen options' and check it') then the image editor will appear. Browse to the image you want to use, via that, in the usual way, and then when you've found it, click the 'featured image' and when it appears in the module (outside the edit box) just close the image editor, and save your post as usual. An image that works best with featured images at least 210 wide.