There are a few steps that you need to do if you enter Japan with a brand new long-term visa (this does not apply if you only come to Japan on a short-term/tourist visa). Unfortunately, no one tells you what you need to do. Even if you were to ask, the probability that someone can answer you in English is pretty low. Therefore, I thought introducing you to some of the steps that are necessary after you enter the country would be a good idea.
When you first come to Japan and receive your resident card, you will be required to register at the city hall/municipal office/ward office (there are many names for places like this, I will be referring to it as city hall from here on out) of the place where you will be living. If you are living in a hotel for a few days, you do not have to register that at the city hall, but if you have your permanent residence, you will need to register that you are living there. When you first go to the city hall, you will need to bring your Resident Card, as well as your passport. When you complete your registration, you will receive your MyNumber (マイナンバー、sometimes called individual number、個人番号), as well as some pamphlets about the neighbourhood, a calendar that shows when garbage will be picked up and so forth. Afterwards, you can also enrol in the National Health Insurance (p. 2) and figure out whether you have to pay National Pension (p.3).
A few days after you registered your address in Japan, you will receive your MyNumber card. The card will be sent via registered mail, meaning that you have to be there to take delivery of the card. If you are not at home, you will receive a notice from the post-office, asking you to come pick-up the card at a local post office or to give them a date and time when they can deliver the card again. At first, you will only receive what is called a Notification card, a small green piece of paper that has your name and MyNumber on it. If you want the full-fledged MyNumber card, that will have a picture of your face and such, you have to apply for it following the instructions you receive in the mail. Once you have a plastic MyNumber card, you can use it to verify your identity to print things like a certificate of residence or similar things directly at the Convenience Store, without having to go to city hall.
If you move, you also have to report it to your city hall. If you move within the same area, you have to go to the city hall and register your new address. If you move to a different area, you first have to go to the city hall of your current address and inform them that you will be moving out. You will then receive a paper stating your current address and move-date. You then have to take this paper to the city hall of the area where your new residence will be, fill in a notice of moving in and hand the two papers together with your residence card and MyNumber card to the people at the relevant counter.
And there you have it. Going to city hall can be somewhat of a hassle, but it is an unfortunate necessity of life in Japan. Especially if you are moving around a lot in the country, it can be hard to constantly update your address but at least the first registration is strictly necessary and if you fail to do that there might be consequences. If you do not update your address regularly afterwards, you might miss important mail regarding your insurance or pension, but that is about it. Still, if at all possible, I recommend updating your address regularly as you move around the country, if you are seeking employment and are living at a different address than the one you are registered at it might make a bad impression. I hope that this was useful to some for you, until next time!