I need a little help understanding how to add a custom

I need a little help understanding how to add a custom sidebar to my store pages. I've read Themeing MarketPress, but I'm not sure I grasp how to simply replace the exisitng sidebar, which i believe is the default template for my site (by the way how do I edit that?).
I simply want on the sidebar for the store, product list, and product pages to have : the cool shopping cart widget that changes dynamically like on the SimpleTheme, and a category list, and maybe a place for specials.
Sorry to be so dense, but i can't seem to get started on this.
~mike