[TYPO3] How do I specify which page ID is the default?

[Dan Temple]

Probably a silly question.

I have worked my way through MTB tutorial part 1 and have much of my
site built. It all works fine when I access the FE like this:

   http://www.my.domain/index.php?id=3

But if I just use http://www.my.domain/ I get:

   No pages are found on the rootlevel!

How do I specify which of my toplevel pages is the default?

This is probably so obvious that it is not mentioned anywhere - any help
appreciated..

/Dan