https://www.gamespot.com/articles/nintendo-switch-online-service-is-now-live/1100-6461869/

Nintendo Switch Online, the first paid online service from the company, is now available for purchase through the Nintendo Switch Eshop. Signing up for the service grants you access to online play, cloud saves, and a selection of NES classics.

To sign up, first you'll have download and install the 6.0 system update. Then you can go into the Eshop and find the new Switch Online tab. That will show the various subscription periods along with the seven-day free trial. If you decide to pay for a subscription, it's priced at $4 for one month, $8 for three months, or $20 for 12 months. A 12-month family subscription is also available for $35. If you don't want to pay out of pocket, you can use My Nintendo points.