a 2D Online RPG to the rank of 100 users online. The graphics will be top-down 2D not isometric, the tile-grid in 64 pixels and the only control directions are UP, DOWN, LEFT, and RIGHT. I know this is just details but it's just the best way that I can use to explain you what I want. Because as I said i'm not an expert and really don't know all the stuff and things that handle a Server Networking.

I apologize for the late response, it has been one of those weeks...
I did some digging for tutorial series around Java or C# since you listed those specifically, and oriented on 2D - multiplayer.
Several series seemed likely to do a beginner more harm than good, but I found one that was presented clearly enough that someone with some familiarity with Java should have no trouble.
Java 2D Game Engine Development
It covers the range from spritesheets to UDP/TCP and packets for multiplayer.

I only watched two of the videos in the series, so if it leaves much out there are a couple of series at The New Boston that teach Java from scratch and basic Java game creation.

While I know you could find tutorials easily enough, I don't doubt you've looked, and come across some of the confusing, broken, messes I found before this one - and were discouraged by them.

Hopefully this one will get you to where you want to be.
At the least it will give you some of the basics on the networking framework and dealing with animations.

//
//

As to the discussion on what constitutes an MMO...
The "Massively" aspect was originally a tragic failure of grammar referencing the design orientation of a game.
A game was Massively-Multiplayer if it was designed entirely around the multiplayer aspect.

Games like NWN and its original hard coded support for an amazing 50 users! ensured the shift from one of Quality to one of Quantity.
Later editions going into the hundreds, T4CY breaking 100, UO breaking 500, and EQ making the 1k mark look trivial, ensured that change and started the gradual increase in values.

..
I understand the frustration of seeing dozens of people asking (often trolling) how to make a game like World of Warcraft when they can't name a programming language, or seem to understand even the laymans' meaning of "condition" but negative responses only ensure it is fodder for the trolls, it doesn't help those asking legitimately to find inspiration to work to their capabilities.

Remember, many who are asking these questions are uninformed enough to still think that learning a programming language is the 'hard part.'
And remember why it is we give time to answering them.

So for me, a game needs both the massive instances AND the persistence to be a true MMO

This is a good example of how subjective the term has come to be.
From originally referencing games with a maximum (coded) capacity under 50, to games not being worthy unless they support hundreds && persistent game environments.

Or the more common usage among those who are not on the development side - a game that is played through the internet by more than one person at a time.

I consider it a great failing on our part as a community that we have not replaced these older terms with more accurate, reasoned terminology and seen it through to common usage.

As someone mentioned earlier, part of our jobs is (now that Systems Analyst is an obsolete profession) to establish viable, fruitful communications with the client or design-team. By not giving them terms that can more accurately describe their intent, we have collectively failed in that regard.

Let's get back on topic.

Look at the REAL question that was being asked. It has nothing to do with MMO, even though he used those words. Also he pointed out he has a language barrier, English is not the native language of Argentina.

Yes, we all know a beginner will not make an MMO. This is For Beginners, and for better or worse, beginners throw around the term MMO when they think about their dreams.

Remember the additional rules for For Beginners. They include, "This is a forum for beginners. Do not flame users because of their lack of knowledge. We all had to start somewhere. This forum is for beginners to ask questions without being harassed because somebody more experienced thinks the answer should be obvious. Make sure your replies are helpful and guiding the beginner in the right direction, not taunting or flaming or insulting them."

Reading it fully, the original question is: "I want to learn how to make some 2D online games. I'm thinking about learning libGDX or LWJGL. Is there something better, and can you please point me to tutorials?"

I think that was addressed through answers by menyo and Looniper, pointing out tutorials related to libGDX and Kryonet for making 2D online games with the tools and background described.

Let's get back on topic

Sorry for my part of that.
It is one of those things that is likely to come back around as often as the term comes up when referencing someone's goals.


To JS_means_JackSparrow
The more 'beginning' series from The New Boston I mentioned is here.
Java Game Development

(frob-Liked the book btw, I'm not a Unity fan and it got me through two projects)

Thank you again Looniper for the time spent.

First I need to tell frob that you're right, in Argentina we speak Spanish. But the second most used language is English, and we learn it from the beginning, at primary school. I know the way of my expression can be seen as bad but I'm very good at reading or listening, much better than talking or writing so I really have no trouble to follow along a video tutorial or a book.

Then, thank you Looniper for the two tutorials, before you posted that, I've started working with this: https://www.youtube.com/playlist?list=PLRIWtICgwaX0u7Rf9zkZhLoLuZVfUksDP

it's a very very well explained Java 3D tutorial using OpenGL. Sometimes I find it a little bit difficult to follow for example with things like Matrix and Shaders, but I'm learning a lot and it's being very useful for me. So when I finish this one that I started I'll begin the ones that you gave me. Thank you!

By the way, I'd like to say that in my opinion a game is considered Massive Multiplayer when it has literally a lot of players online simultaneously, like 50 or 100 or even more. The problem is that most of you are considering the new fashioned MMORPG's that are out there maintened by giant companies as the main examples for that terminology, and that's wrong. MMO's are not only World of Warcraft, there are a lot of indie games out there that can support more than hundred of players and are not maintened by big enterprises. So start opening your mind and take off your mind the concept that a MMO means 3D game with 100 channels with 10000 players, with 5 billion revenue and an extremely huge datacenter.