I'm having trouble centering this image. Also my sidebar doesn't seem to exist.
Centering Pages, Images and other elements with CSS
In that case, why don't you start your project(s) with one of the pre-built CSS Layouts in DW?
Go to File > New > Blank page > HTML > Layouts. Select one of the fixed-width, centered layouts from the 3rd panel and hit Create.
The code is well commented to help you understand what each bit does.
Look at Dreamweaver Templates. It will give your site a consistent layout that you can use to spawn new pages.