Its going to be challenging to provide you with actual code based on your description. If I understand what you want is to show data from a specific table depending on the value of the dropdown item that is selected. You should be able to do this from the code-behind (event will depend on if you want to capture the item on post back, or will you require the user to click on a submit button, etc.).
For help with asp.net, I always recommend that people start by accessing Microsoft's official site: http://asp.net