Saving a game is very simple. You have to just press shift + s and the game is saved.
To load the game, you just have to type the same name as in the saved file. If you use different names, then you can play multiple characters simultaneously.
You start the game and put "John Doe" as your name.
After selecting your race, class, gender and alignment, the game starts. You can now save the game at any time you want by pressing the shift and s keys simultaneously. The game will ask if you are sure you want to save ("Really save? [yn]"). If you press the "n" key, the game will continue. If you press "y", the game will save and quit.
When loading the game again, type in the name you used for the saved game to load the save. In the "Who are you?" screen, type "John Doe" without the quotation marks, and you will load the game you saved with that name.
On Unix systems, your NetHack player's name is your username by default. To use a different name, use the -u switch to nethack like thus:
$ nethack -u Jack
This will either start Jack's game or load Jack's saved game.
The /dev/null/nethack tourney rules do not permit you to have more than one account, but you can have multiple simultaneous games by having each game running on a different server.