Google Sites now offers Site Templates to get you started with a structure and a good looking theme. Google Have also provided Template Tips site to help you change your site to how you want, as there may be some elements of the Templates that you do not know how to edit.
If you want to be able to change the logo at the top of the page (the area that has "Name" followed by "your tagline goes here") in the header you can download the logos, in JPEG and Photoshop PSD format, from the Template Assets page.
The Template Tips page gives comprehensive instructions on each area of the site so you know which part of the template is which. The Tips also include a set of instructions on how to change the content on the site so you can remove the default content and add your own - see the Topics list on the right.