jr pass

how do we use jr pass after we exchange. do we need ticket before we board? i assume for reserved seating we will need to get at ticket. but for unreserved do we just board and find a seat?

Your assumption is correct!

Indeed, if you want to make use of the free seat reservation option, you can go to any JR ticket office and join the line. I always recommend people to write down the names of the towns and date and estimated departure time as it makes things usually go a little smoother if you don't speak Japanese. Note that you can make several reservations at the same time. A reservation ticket contains the date and time, train number, car number and seat number. The platforms usually indicate the car numbers so it is real easy to find the location on the platform before the train arrives.

For non-reserved seats you'll need to find out what car numbers are the cars for non-reserved seats and you'll need to line up with the others. Note that often actual lines are drawn on the platform to help the queueing process. You'll find that often several lines are drawn to aid people already lining up for the trains after the next. Complicated? It all sort of makes sense when you are on the platform :-)