As Amalthea said , Sumimasen and Arigato Gozaimas will cover a lot of things . Here's a few more :
Stand to the left on escalators . The right side is for passing . The same goes for sidewalks and building entrances, but this is not always strictly observed . Go with the flow of the crowd .
Bicycles on the sidewalk may ring their bell when coming up behind you . Move out of their way when you hear it .
In convenience stores , the norm is to wait for the cashier to bag your purchases and hand them to you before you give them the money .
You do not need to bow to store workers or restaurant personnel , but a slight nod of you head is a nice way to acknowledge their politeness to you .
If there is a small , flat tray at the cash register , place your money on that ( observe what the customer before you does ) . Some places do not have these , so you just hand your money to the cashier .
Most stores and restaurants will greet you with " Irrashaimase " ( EE RAH SHY MA SAY ) . This means Welcome to the Shop , and you do not need to rely to this with anything except a smile . Grocery stores and 100 Yen shops are self -bagging , which is done at a small table close to the registers .
A few nonverbal clues : Any tilting of the head while sucking air through clenched teeth or muttering the word " muzukashi " means NO ( literally : it is difficult ). There is no argument for this phrase .
If the matter is important , your only hope is to look worried and muse out loud " What shall I do ? " You may get a workable suggestion . If none is forth-coming , you are out of luck .
The actual word for NO is " ie " ( EE-EH ) , which ironically sounds much like Yeah .
Crossing one's arms in front of the body to form an X is the intenational symbol for " It is forbidden " . This is usually used as a No Entry indication here .
When boarding a train , stand to the side of the doors until arriving passengers have gotten off .
If you think you will have trouble with chopsticks , feel free to take your own fork to restaurants . It is not rude , although many places have both cutlery and chopsticks .
Regarding the Big Beautiful people : Japan has its share of them . Do not feel uncomfortable ! The furniture here is no more fragile than in any other country . True , you will have a tough time finding clothes and / or shoes . I am a US size 8 and that makes me an Extra Large in Japanese clothing .

I understand that your Japanese friend will be reluctant to correct you , but if you make it clear to her that you want to learn , then that will put things in a different light for her . She will not be correcting you , she will be
teaching you .
When you say to a Japanese person " Please teach me ." you will open up wonderful doors to their culture !
That said , you are an honored guest in their country . Short of committing mayhem , you can do no wrong . They know you are not in your element . Be free with compliments about their country and ask a lot of questions and you will have an experience that you will treasure for the rest of your life !