This playlist with a new player being led by an experienced player was very helpful for me when I was learning to play. Still haven't ascended, but what an addictive game.

https://www.youtube.com/playlist?list=PLHzN3MktVPzVdFNz7WYwG...

Official guidebook for basic knowledge http://www.nethack.org/v361/Guidebook.html

For more advanced mechanics & spoiler http://www.steelypips.org/nethack/

There is also a NetHack wiki https://nethackwiki.com/wiki/Main_Page

Public servers like NAO is also a good place to learn NetHack. https://www.alt.org/nethack/

You can download session files from players and play it on your machine, their configs are also available on the website.

I'm a Dungeon Crawl Stone Soup player, which is a cousin to Nethack, but I imagine the process is the same: get an IRC client, connect to Freenode, join #nethack, ask questions, and learn to use the utterly invaluable resources provided by the in-channel bots (e.g. the learndb).

NetHack's learndb (Rodney) isn't quite as vital as Crawl's, mostly because NetHack is more spoiler-reliant and (as a result) has a much more detailed and accurate wiki, as well as other sources of spoilers.