You will be given a ticket which states the name of the train, the car number (=cabin number), seat number and the time of departure. That's the most important four elements to take note of. The ticket will not state the platform number. You will have to figure it out from the electronic boards so you need to know the name of the train you are taking.
For example, if the ticket states Narita-express 8, it doesn't mean platform 8. "8" is part of the name of the train (they differentiate the trains by their timing so 8 means the eighth train of that day). The cabin number is usually stated on the electronic board on the top once you have found your right platform. (The picture belows show cabin number 8).
指定席 (Shi-tei-seiki): Reserved seat
After riding the NEX, you will most probably be catching a shinkansen from Tokyo. Shinkansens are pretty similar to NEX except that the leg room is less. You can't afford to squeeze your luggage into that pathetic space infront of you so park it in the luggage compartments. It is common for Japanese to buy bento (lunchboxes) for the long ride on the shinkansens. Buying your lunch box outside the station is cheaper than in the station and in the station, its cheaper than those selling on the platform. With convenience you need to fork out more.
Next would be the local trains. For most of the local trains, the door has to be manually operated. You will have to push a button for the door to open when you want to board. After boarding, push the "閉" close button. Likewise, when you want to alight, push the "開" open button.
Commonly used Japanese phrases when riding trains
Please give me a ticket on the 2pm train to XYZ: XYZまで二時発車の切符をお願いします
Pronounced as: XYZ ma-de ni-ji ha-(short pause)-shya no kip-pu o o-ne-gai-shi-mas
Where is the boarding platform for this train?: この電車の乗り場はどこですか。
Pronounced as: Ko-no den-shya no no-li-ba wa do-ko des-ka
What time is the last train?: 終電はなんですか。
Pronounced as: Shuu-den wa nan-ji des-ka
Does this train goes to XYZ?: これはXYZ行きの電車ですか。
Pronounced as: Ko-re wa XYZ e-ki no den-shya des ka