There must be more to your question than I am reading. When a user drags the mouse over text in a textbox it is highlighted. If they then right click there is a default popmenu associated with the textbox that has cut, copy, paste, delete.
If you are giving the textbox focus programatically and not via the mouse, you can simply set its text to an empty string:
Private Sub Form_Load()
txtMyTextBox.Text = ""
End Sub