Newbie here, just get my Nintendo Switch, this is my first Nintendo Console, read many post related to eshop but i still very confuse, can any one please help to point me some guideline how to setup Nintendo Account, eshop, payment method, etc.
Another question, just notice Mario+Rabbits battle kingdom has new DLC to be released soon, if I buy Mario+Rabbits battle kingdom cartridge, how to download or buy the DLC? Must have eshop setup?

Just go to "my nintemdo" and create account
When creating account choose us/eu/jp/mexico or whatever eshop available country (you can change country later if you want)
After that login eshop, payment method visa mastercard also ok so far, you can buy top up card from legit store

For mario rabbit, if you bought non DLC included version you need to buy on eshop.
DLC included version should be gold color and has extra title gold or deluxe whatever version
Before buying DLC make sure your game card country version and eshop country are matched for safety...