If you want a booklet: Set up a new document with Print intent, and the page size A6 - decide if you want portrait or landscape orientation (with bleed, if you have bleeding content) add the number of pages, (divisible by 2 or 4 depending on the extent) create your design (that is adding text and images), and export it as a PDF with separate pages and trim marks included and bleed, Your printer will advise you which flavour of PDF s/he wants.
An A3 poster would have a different set-up!
if you're going to do much of this kind of work with InDesign, it will be worth you taking a course, such as those offered online by Lynda.com and Trainsimple.com.
Hi Derek many thanks, it's more of a poster, folded to A6, with the map spread over several of the A6 panels.
Try this, assuming you want the (unfolded) poster to be A3 landscape, you can produce a mock-up o show your printer – get an A3 sheet of paper, fold it on the long edge, each time, until it's A6. Open it out so it's back to A3, with fold marks, you can then see the A6 panels and produce a template in InDesign.