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