This is one of the basics. If you know DropDownList you must know how to do it. I guess you are new to asp.net serving you the answer.
TextBox1.Text = Convert.Tostring(DropdownList1.SelectedItem.Value);
TextBox1.Text = Convert.Tostring(DropdownList1.SelectedItem.Text);
or
TextBox1.Text = Convert.Tostring(DropdownList1.SelectedValue.Text);
TextBox1.Text = Convert.Tostring(DropdownList1.SelectedValue.VAlue);