What do you have in terms of WA extensions?
there are ways to do this with and without them, but often, asking direct questions with respect to an extension can get meaningful answers more quickly.
As you know, your templates allow you to determine how your page looks, and where the data goes.
In order to display your database information on your template based page, you need to create a connection to your database.
once this is done, you create recordsets, which filter the data from your database, according to the parameters you set.
You can use dreamweaver to display this data, or use dataassist to help make the process a bit easier.
in your scenario, it seems like you want a products page to list your products, which can be filtered by category, depending on the category chosen from the menu.
this would link to a product detail page, depending on the product chosen, where the single product item would be displayed with all relevant images and details. This page is where you would add your add to cart behavior.
I'm pretty sure there are walkthroughs using the blue sky music database as an example, within the ecart dociumentation.
Perhaps a member of the WA team could help you with this
take a look at this thread too... showthread.php?t=7814