Narita express from Tokyo station

I want to catch the Narita express from Tokyo station, what is the easiest way to find the Narita express line and can I purchase tickets at any counter ?

You follow the huge well written signage to the platforms to catach the N'EX.

Yes you can buy a ticket at the counter.

Many people seem to think that the Narita Express is somewhat special, but really it's just another JR train. You can buy tickets at any JR ticket counter as you do for all JR trains.

Save yourself some time and buy your ticket at the ticket machines.

You can buy the tickets at major JR stns multiple ticket vending machine, ticket windows at other JR stns, and also online(conditionally).
https://youtu.be/PbS0J-2UIYw
If you do not have enough time to buy a ticket, you can enter by IC(Suica etc.) and buy a limited express ticket from the conductor in the car.

Look the departure sign board at the ticket gate and move to the "Sobu Line" underground platform.
https://www.jreast.co.jp/e/stations/e1039.html
https://www.jreast.co.jp/E/stations/img/map_e/e1039.gif
This is Marunouchi(west side area name) central ticket gate to B4F(not platform/B5F).
https://youtu.be/Q4YdEgdbmeE
This is other side Yaesu-guchi(east side area name) central ticket gate to B4F(not platform/B5F).
https://youtu.be/najss8InBV0

After 14 plus trips to Japan I still cant come to grips with ticket machines, though I have finally cracked it for local trains. I find it easier to just go into the ticket office for the NEX tickets.