The process of creating printer spreads from layout spreads is called imposition.
InDesign’s info about imposition can be read here.
Before you print, you’ll have looked at your booklet as reader spreads. That is, the spreads print in consecutive order and can be viewed for edits and design changes. However, if you try to fold it, the pages will be out of order.
If you’re printing at home, you’ll save your document with InDesign’s InBooklet feature. Click the link to learn how to do this in CS5 (for those of you working in CS4 and lower, it’s much less of a headache—lucky you! Once the booklet is imposed in to printer spreads, you’ll print first the odd pages, then flip them and run the even pages through. Voila!
If you’re printing at a service bureau like Minuteman Press etc., save your document as single pages and let the bureau impose it. Any reputable printer should have software to impose your PDF with ease. If your printer asks for imposed files (I’ve had a few students tell me that their printer does), than I recommend going to another service bureau. That is a basic part of THEIR job, not yours.
Phew! If you’ve read through this and are still confused, come and chat with me.