My own database into Commerce Server 2009-Collection of common programming errors

Questions you need to ask yourself:

Do I want to create a Generic Product Definition or specific Product Definitions for my products?
    A generic Product Definition will allow you to quickly load your data using the same product definition.

Categorical organization of your data on the site?
    How is it going to be organized? Do you have enough information in your database to create this structure and place the Product at that location?

You should get use to using the Catalog and Inventory schema manager. You will need to define all of the properties that you want to use on the site. Organize this properties into appropriate Product and Category Definitions. The sample Adventure Works Data will provide the basics on this approach.

Once you have this, you can leverage the Catalog APIs to load your data or the Catalog Manager to import the data in an XML format. Ravi’s tool provides a quick view at how simple the APIs are to use. They are quite flexible as you can do all of the schema and catalog management with the APIs.

Enjoy.