The JR train from Narita To Tokyo is called the Narita Express.
The platform for the JR Nartia Express is on the lower floor of the airport, follow the signs to "Railways" which will bring you there. Look for the JR trainline ticket gate, should be pretty easy to find.
You don't need a reservation, there's sure to be unreserved seating available, the unreserved carriages should be marked clearly on the outside. Or if you prefer, you can get a reservation from the ticket office even just minutes before departure.
Also, cheapo tip from our article on this:
Given that it doesn’t cost that much to get into Tokyo, you should think very carefully before you activate your JR Pass especially if you plan to spend the first few days of your trip in Tokyo. If you’re spending a few days in Tokyo first, you might want to choose another option to get into Tokyo and then activate the pass when you embark on a longer inter-city trip (i.e. you can just buy a normal train ticket).