You have to figure out first which helping organizations still accept volunteers. This is compiled on this page (google translation) (in the table 1/3rds down the page).
The primary challenge might be that you will have to provide transport and housing yourself in many cases. The minimum I would prepare is an international drivers license and a sleeping bag.
The next one will be that you need insurance for what you do. Since it might require you to also help building a wall etc, and you are injured, you get yourself and everyone else into huge trouble. Some of the help centers apparently provide a insurance scheme, but I am not sure if they do so also for foreigners. Getting an insurance from a Japanese company if you do not have a residence in Japan is basically impossible.
Now, the third issue is that only one of the pages on the list actually offers an English page, and it does not seem to specifically mention counseling help possible through foreigners and rather offer all services they do, but they do list these phone numbers:
For more information in English,
Call: +81-198-62-1001 Tuesday through Sunday
Call: +81-3-5809-3953 Monday through Friday
English operator may be available
Since I do not know your language skills and neither know what language skills are required for the individual jobs, it will be hard to make any recommendation for you what you should know or learn for such a job. I would recommend that you contact this one service and talk to them first in English, and then show how much Japanese you know. They will then be able to assess, what job is best for you to do.
One of the jobs they offer, which seems to fit most to what you want to do is this here:
Listen to stories of afflicted people while massaging their hands for relaxation
Regarding the other services on the list, I would suggest running through the pages with google translate and seeing if you can find any information about foreigners having worked there in their activity logs. Then, contacting them directly might be the best idea. If you cannot make yourself understood on the phone, you know that this will not be the right place for you.
Otherwise ask someone in Japan you know to help you contacting the services and ask them how you can be of help.