How to Get a JR pass
         You purchase a 
Exchange Coupon prior to arriving in Japan, I use 
JTB Travel  but there are a other sites offereing them.Allow two weeks for it to be delivered. 
You cannot purchase a JR Pass in Japan.
         Once you arrive in Japan you go to the Shinkansen Ticket Office in the city you are staying (assume landing in Tokyo or Osaka) and exchange you a 
Exchange Coupon for a JR Pass
         
You can specify the dates you want to activate it . 
If you have a few passes for a few different dates , exchange them all and set the dates at that time