Beginner's SqlDataSource SelectCommand Question

Hi folks,
I'm having problems using the SqlDataSource to return certain values from a SQL database. I have two DropDownLists. When the first DDL's selected index is changed, I need to take the value (an int id) from the field, use the value to look up a string in a different colum in the table for that record id, which should then be used to automatically select a value from the second DDL. I think my problem is I'm not sure how to get the SelectCommand to actually return the value, let alone in string form!

Here is my code:
{
        string selectedProject = ProjectDDL.SelectedValue;      
        SqlDataSource temp = new SqlDataSource();
        temp.ConnectionString = rootWebConfig.ConnectionStrings.ConnectionStrings["DBConnectionString"].ToString();      
       
        temp.SelectParameters.Add("id", selectedProject);
        temp.SelectCommand = "SELECT [username] FROM dbo.projects WHERE id = @id";              

        //temp.Select(); ?

        //AssigneeField.SelectedValue = string returned from Select Statement!
} 
 
I'd appreciate it if somone could point me in the right direction?
 
Thanks,
Ally