I never played this games, it gets a little discount. But acording to PCgamingwiki, it doesn't have the DRM. But how the multiplayer works? And what mods shoul I use?

By default to works via Battle.Net 1.0, which doesn't require login but simply create an account based on your hero name.

I would recommend using DevilutionX, it fixes many of the bugs in the game, has lots of QoL and allows playing directly via ZeroTier (with the release coming this weekend) with our requiring any setup.
Note that I am a developer on DevilutionX, so of cause I like it :)

^ that

and if you get tired of Diablo, try using The Hell 2 mod.
It's the biggest overhaul mod for this game, heavily biased towards deep character customization.
https://www.patreon.com/thmod

And what is the best class to start?

Rouge

I agree with Anders, as the Rogue can pick hostiles off from a safe distance, while you are learning the game and its mechanics. IIRC, I had a bow that could do Knockback, which was very helpful, perhaps even over-powered. That was my first play-though, waaay back in '96, long ago. Playing this game back then with that atmosphere with the lights off sure was a good time!

Then later in life when I played it again years later, I did a Warrior. IMHO, this was the hardest class out of the three classes to beat the game with, as the Warrior has to get up close to whack hostiles, which makes you get whacked right back. But I had a melee weapon that could do Knockback, which sometimes almost felt like cheating.

When you've played just long enough to understand the game and its mechanics, I think the Sorcerer is the way to go. Building up your spell book and utilizing the spell variety made the game waaay more fun IMHO. I have no desire to repeat the game as a Warrior or Rogue, but there's always the itch to build up a Sorcerer again.

We have community that still plays classic Diablo 1 here on Discord https://discord.com/invite/AckTPMu there is a web as well http://thehoradrim.ddns.net/home/

Hey Moncavo, just wanted to let you know that DevilutionX 1.3.0 is now avalible. Including multiplayer with no need for router configurations, and a Portuguese translation, hope you will enjoy :)

So Battle Net for D1 is dead, right? I can't even create a game there, 'cause of the "6112" msg, but I already opened that door...

Yeah form what I'm seeing it died back in May so DevilutionX might be the only option for public multiplayer now (I don't know if Game Ranger is able to work on it's own).

Gameranger can work on its own because it only emulates LAN, but using it is pointless when DevilutionX has the best multiplayer around ;)