(the following applies to enhanced edition)

Playerwise, the main servers (main as in where most players connect, not necessarily the best) are Arelith and Ravenloft, but you can check status and playercount here: https://nwn.beamdog.net/

Both of them are role-playing servers, you act/speak as your character at all times (it's far less daunting than what it might seem)

Ravenloft is gothic horror themed, think Curse of Strahd from pen and paper. https://www.nwnravenloft.com/

Arelith is based on Forgotten Realms, in/around an island far in the west. https://nwnarelith.com/

Both of them have a lot of custom mechanics, classes, ... not found on the base game. Also, both of them are big. They are probably over 1000 different areas (as a reference, the full base campaign, The Wailing Death has less than 100 areas (from the top of my head))

In case you want to try Arelith, it's split between a few (interconnected) servers. You probably want to check the Distant Shores one. It's designed for new players / low level characters.

There are also some action oriented, no RP required, just kill & loot if that's more of your thing. There's one I personally liked but it's been offline for a while, hometown diablo. I also tried one based on middle earth for a while.

Just check a few (smaller servers might seem "less lively" but DM run events might be more custom tailored) and see which one you like most.

To connect, just click on multiplayer and connect to a server. It might take a while as the game will download all the custom content used in the server.