In Japan, almost no on-ramp payment via apps available, all you can do is buy one at the Exchanges certified by the government, and transfer it into your self-custody wallet. So the answers are:

- Guides: Follow the instructions of Exchanges certified by the government

- Where to buy: same as above

- How to store:

- If hot one: Blockstream Green, Sparrow, Electrum

- if cold one: Jade or COLDCARD