Rail pass and reserved seats

Next month I will arrive at Narita. I will have a Japan Rail Pass and I will take the JR Narita Express to Tokyo but I still need to reserve a seat, is that right? Where and how do I go to reserve a seat? Same question for the Shinkansen.

You can book your seats both for Narita Express and for shinkasen at JR counter at the Narita Airport. (basement floor)

Show your coupon and passport to the staff to activate the pass at the counter.

Buckie,
In your home country you bought the Exchange Order for the JR Rail Pass. This Exchange Order plus your passport you show to the JR staff at Narita airport. They will then give you the JR Rail Pass starting the day you mentioned. In your case day of arrival as you want to use the Pass immediately for the Narita Express. So you say just "next Narita Express to Tokyo ( or Shinjuku, Ikebukuro, Shinagawa ) and you will receive a second ticket what is your seat reservation.
Same goes for shinkansen rides: You choose any of the shinkansen, but no nozomi type. Here you have the choice. Just enter one of the "non-reserved" cars and take a free seat. ( Not valid for hayate and komachi typre shinkansen = only reserved seats ) Or you go to a JR ticket window at the station show your JR Rail Pass and you will get a free seat reservation. ( Also valid for hayate and komachi shinkansen ).
Same goes for JR Limited Express and JR Express trains, you can choose reserved seat or unreserved.
But do not waste days of the validity of the JR Rail Pass just for the Narita - Tokyo leg or within Tokyo. The JR Rail pass saves money on long distance train travel on JR trains not on inner city commuting rides.

I'll second Peter. You don't need to waste one day portion of your JR pass just for transport to Tokyo. You can have the starting date postdated up to one month ahead when you get your pass validated. The JR counters at Narita Airport are not very busy, so it might be a good idea to get the pass activated and make reservations for the Shinkansen there.

After the transaction, I might suggest you to head to Central Tokyo by Keisei Line. The fastest and very comfortable Skyliner costs 1,920 yen and takes 55 minutes to Nippori, where you can change to Yamanote Line. Their limited express costs only 1,000yen, with additional 10 minutes ride.

I second the previous persons
let's say that you have a 7days JR pass and you will stay 3 days in Tokyo (not counting the arrival day) then travel, for argument sake, to Hiroshima, Osaka,Kyoto etc and back to Tokyo all within 7 days, then stay few more days in Tokyo, Nikko etc. before departure.
you will exchange your voucher at arrival in Narita airport but ask the staff to start you pass on the day you leave Tokyo for Hiroshima, 4 days later. At the same time reserve your shinkansen seats for your trip to Hiroshima. you could reserve all the seats in advance for the whole 7 days trip too but this would be to inflexible. Reserving the night before is more than enough.
for your first trip to Tokyo from Narita take one of the Keisei trains then for sightseeing in Tokyo buy day passes (check on travel, transportation etc. at the top of the site for all the various passes. This also means that you should preferably get a hotel either near a Keisei station or near a Yamanote line station to avoid dragging your suitcases around too much this first day.
I hope that we aren't confusing you too much.. if you are used to long distance train travel on European trains this is quite similar.

Thank you for your most helpful answers
Buckie