Go to File > Page Setup > Layout. Under 'Publication type' select 'Booklet'. If you normally use A4 paper you should find that the correct dimensions are automatically selected and that, by default, landscape format is also selected. Click 'OK'.
You'll be asked if you want to add 3 additional pages. Select 'Yes'. You've then got a 4 page booklet, made from a sheet of A4 folded to A5 size. Page 1 is the front, pages 2 & 3 are in the centre, with page 4 on the back. (You can move between the pages by clicking on the relevant number at the bottom of the screen. (If you want your booklet to have 8, 12, or more pages, go to Insert > Page and add the extra pages).
Getting booklets (larger than 4 pages) to print out properly can be a bit tricky. (The Publisher help file says that you need to use a photocopier - or the photocopy function of a scanner - to get the 'back to back' printing done. Some printers facilitate 'duplex' printing but you often end up having to feed the paper through twice to get the correct sheets back to back). A bit of trial & error should guide you to the best way to do it with your printer.
Chris