This is nothing complicated, I'm assuming you are trying to use a
DropDownList to prefill a TextBox so people don't have to type common
things.

What you need to do is create a DropDownList with the AutoPostBack
attribute set to True, and then handle it's SelectedIndexChanged event
to set the Text property to the value of the SelectedValue property of
the DropDownList:

<asp:DropDownList ... AutoPostBack="True">
 <asp:ListItem Selected="True">Pre-typed things</asp:ListItem>
 ...
</asp:DropDownList>
<asp:TextBox ... />

Here's an outline of how you would handle the event:

Sub DropDownList1_SelectedIndexChanged(sender as object, ...)
Handles ...
 TextBox1.Text = CType(sender, DropDownList).SelectedValue
End Sub

I'm not sure what you mean by updating the database, it's unstated
whether this is a new record or editing a record, if there is already
existing code that this will work alongside, etc. You will have to
explain your problem further.

-Michael Placentra II