The JR Pass comes in a few forms. National and Regional. With the national JR Pass you have the option of purchasing 7, 14, 21 consecutive days of use. I bought mine from H.I.S Japan UK. They’ll issue you an exchange order, it is very important to ensure you do purchase within 90 days before the intended day you exchange your voucher when you’re in Japan. Any JR Pass can be exchanged at any JR Offices at airports and major stations. It is valid for all JR Lines, some trains however will not be included with this pass i.e. Nozomi and Mizuho.
I’d mainly use the pass for travelling to and from Kansai to Kanto (One way already costs 13,620 Yen for a non-reserved seat and it’s 14,200 Yen for a reservation) then a few trips to other places via the Shinkansen and JR Lines, it works out cheaper for me. Especially when knowing it’s not all the time that I would use this pass, sometimes I would have to take the subway and bus which the JR Pass can not be used for.
Reserving a seat is very easy, just pop by the ticket office at the station and if you already know which train and time you’re getting on then you can just simply tell them. If not, they have a screen with all train times and destinations so they’ll usually ask if you want the earliest departure train, followed by your preference of seat (Window/Aisle) Of course, reserving a seat is optional and I’d always prefer just to simply hop on! depending on which Shinkansen you’re boarding they have designated carriages for non-reserved passengers, typically 1-5 or 1-3.