How to order at Japanese Shokudou?

Hi there, I have extreme anxiety thinking about ordering at any Japanese food stores and even trying to buy things from the supermarkets. What should I even say like when I'm trying to order food? What if I don't even know what to say to order food? Should I just use "Kore kudasai"? Anything else I need to say?

Point at an item in the menu or a wax model in display case, and optionally say "kore kudasai". At some restaurant, you have to buy the ticket at the vending machine (usually located by the entrance) and hand the tickets to the staff.

You can find sites and vids similar to this.
https://goo.gl/bXLxHm
Although there is no English on this site, but easier to study because there is sound files.
https://matcha-jp.com/jp/2635
Use Google translate, but I do not know if it is a commentary for Germans,
so some Roman spelling for speaking and some translate are not and may not be correct.

The place where the ticket vending machines are located is prepayment system.
Just give it to restaurant staff while saying "Onegaishimasu. =Please.".
Such vending machines in Ramen, Exp(high)way parking SA/PA and city/town hall restaurants mainly.
https://youtu.be/ybfUbMFVHYw

Self-service like university cafeteria is also available in lower price restaurants.
She is a Taiwanese international student and speaking Chinese, sub is Japanese.
https://youtu.be/w_mpaTjYWx4
She is a student at this university, so she uses a special prepaid card for payment.
In your case, it will be in cash like this, this is in cheaper Udon restaurant.
https://youtu.be/iUj9c1A_kw4
She is also Taiwanese and speaking Chinese, sub is Japanese. (same channel)

At supermarket is same,
- Take a basket or cart at the entrance.
- Pick up items.
- Payment at cash register. (larger stores are using with self and semi-self cash register usual)
https://youtu.be/Ornh0b_4WqE
https://youtu.be/lvdrkpDqqiU

Basically, you do not have to worry so much, "(This) one pls!" in Engligh is possible mostly,
it is the worldwide common language to show the number and intention with fingers.
For a temporary trip, you do not have to remember how to read numbers.
https://en.wikipedia.org/wiki/Japanese_numerals

If you want to use Japanese, "Kore wo (number) kudasai"
or "Kochira wo (number) onegai-shimasu" is more polite than "Kore kudasai".
There are polite words in English, but in Japanese it is finely divided according to the level of politeness,
you can not remember all perfectly in short period.
https://en.wikipedia.org/wiki/Honorific_speech_in_Japanese
https://en.wikipedia.org/wiki/Manual_keigo