From Nintendo.com :
Online Play on Nintendo Switch
You’ll be able to play compatible co-op and competitive games online by signing in with your Nintendo Account. Online play will be free for Nintendo Account holders until our paid online service launches in 2018.
After the free-trial period, most games will require a paid online service subscription from Nintendo in order to play online.
This service is only for Nintendo Switch. It does not affect online play or features for the Wii U or Nintendo 3DS family of systems.
Online lobby and voice chat
Our new dedicated smart device app will connect to Nintendo Switch and let you invite friends to play online, set play appointments, and chat with friends during online matches in compatible games─all from your smart device.
A free, limited version of this app will be available for download in summer 2017.
Classic Game Selection*
Subscribers will get to download a compilation of classic titles with added online play, such as Super Mario Bros. 3, Balloon Fight and Dr. Mario.
Nintendo eShop deals
Special offers for subscribers may include discounts on select digital games and content.
*A Nintendo Account is required to access online services. Prices are listed in U.S. dollars and do not include tax. Pricing for additional countries will be announced closer to the launch of the service. Subscription terms for the Nintendo Switch Online service will apply and will be made available prior to purchase. Internet required for online play.
Compatible smart device required to use app, data charges may apply.
Currently, the paid online service is available for customers in the U.S., Canada, and Mexico. For the latest list of countries, please visit Customer Support.