The catalog and shopping cart sort of go hand and hand together. There are a lot of premade scripts and tutorials though. www.asp.net has a lot of resources. A shopping cart is not terribly advanced, but it is a tid bit complicated and is a hefty bit of code.
Basically here is how it works:
You output a list of products, it can be sorted, it can select products with a certain value in one feild, whatever. You output witht he products which would idealy be in a repeater a link with a query string to it with the product id. When the user clicks the link he is taken to another page, the query string is requested by the other page. Then the value that was requested is added to the users cookie, if the cookie does not exist it is created and the value is dropped in. Then on the shopping cart page the cookie is requested and that item is selected from the data base. If no cookie exists then when you request the cookie data nothing will show up, so you basically do an if statement to see if anything is there, if nothing is there you output no items selected.
ps: I merged your threads.