Hello, World!

I`m new on VB. I`ve made some ASP pages that works with Access. My problem is this...

When a user will try to add text in a numeric field I would like to come up a warning message when the user will hit the submit button.
So... I know that the code should be something like this:

sub btnsumbit_OnClick()
...................
If (blah blah) < blah blah then blah blah...
Else...

Well... I don`t know how to continue after the "IF" and what to do after the "ELSE"

sub btnsubmit_OnClick()
  if (blah blah) < blah blah then
     msgbox "ERROR"
     exit sub
  end if
end sub

u come out of the procedure.