Nintendo has opened up the ability to register a new Nintendo User ID in preparation for the release of the Switch. However, if you haven't used Nintendo's online services before, this process can be a bit confusing. So here's the basic rundown:
You'll need a Nintendo Account. This is tied to your email and it's the top of the online pyramid for anything to do with Nintendo. You can create a Nintendo Account here, at a menu screen like the one below.
Wii U and 3DS owners will be familiar with the Nintendo Network ID (NNID). This was used to handle anything online for the Wii U or 3DS. While not required, you can link an existing NNID to your Nintendo Account. This will allow you to share your eShop balance between devices. Here's a link to the website, which should look like the image below.
Now we come to the new Nintendo User ID. The details here are a bit vague, however, it seems likely this will function like a Gamertag or PSN ID when you begin playing on the Switch. You create your User ID by logging into your Nintendo Account and simply creating a User ID.
It's a fairly straightforward process. However, there are quite a few details about Nintendo's new online service that are not clear. Many gamers are still wondering if a trophy/achievement system will be added. Nintendo needs to clear these other issues up ideally sometime before the Switch launches, which is right around the corner on March 3.