https://www.cnet.com/how-to/nintendo-switch-online-how-to-sign-up-for-the-paid-multiplayer-service/

Signing up for the service isn't too difficult, but the process takes a couple of steps.

Before you can buy a subscription to Nintendo Switch Online, you need a Nintendo Account. This is where Nintendo keeps track of your credit card information, purchase history and any family group settings you make. If you already have one of Nintendo's older Nintendo Network ID accounts, you'll still need to register a new one here -- that system was retired at the end of the Wii U's life cycle.