Let me see if I understand you.
1) You are using c# and asp.net
2) You have a form (with an extension aspx)
3) On the form you have a dropdown list and a submit button.
How are you creating these pages. I will assume visual studio.
If all the above is correct, then when you placed the button and drop down list on the page, visual studio will have created boiler plate code in the code behind file. (which is mypage.aspx.cs)
If you double click (in development mode) on the submit button, that will take you to the default handler for the submit button.
To access the dropdown list you need something like
Code:
string myValue myDropDownList.GetSelectedItem().ToString();
Hope that helps, if Im incorrect in my assumptions, post back here and I'll try again!
If you post it, then you will be able to retrieve it on the server in teh code behind file.