Greetings,
The page that is shown after confirmation (or immediately upon signup, if confirmation is disabled) will be the "Signup Success" custom HTML page. You can edit sets of Custom HTML pages from the List Settings -> Custom HTML page. Note that if you want a custom page for this specific list you will want to click "New" on the Custom HTML editing page and then, once you save your new set, go back to the List Settings page and choose the new Custom HTML set for use with that list.
Regards