The Nintendo Switch Online service launches on the 18th or 19th depending on your region. You’ll be able to have a week’s free trial from the e-shop on the launch. However there are some urgently important things to know regarding this. From the 18th you will not be able to unlink your Nintendo online account from your Nintendo Switch profile.
Please make sure you have the right one connected before then. This is also very important when making children’s accounts and they cannot be moved between family groups. I made a video showing the issues I had to deal with when making the family group and how to avoid or fix them.
If you need more information regarding Nintendo Switch online you can either check out this video by Nintendo and go onto your region’s website to check pricing.