hi and welcome!
So let's get it started: take a look to the wiki
Somebody said to me long ago, if you want to make your dream real, you must do it yourself. And so, this time came (not so long ago), when I decided, that I want to start making game.
wise decision, in my experience, in game making you will always learn something!
In my own head I already know every aspect of it, how it must look like. I write down crucial aspects of game, that will help you give me right advices, where to start from:
1. It will be based on space setting, where you're flying on ship.
2. It will be 2D with top-down view.
I stop you here. My advise is make a small prototype with first two points: gather graphics, learn slick and be sure about your design choices. Then move and try another frameworks (Artemis or MarteEngine) and decide what do you want.
Then move to other points: be sure about your foundamentals!
3. It will have deep RPG elements.
9. It will have more action during combat and will rely on skills of player.
this point is crucial: define classes, skills, balance them all and be sure about char progressions. It's hard to have a good balance, if you have pVe but also on pvp. Learn from bigger projects and steal, organize your ideas before proceeed.
4. It will have Singleplayer (Solo Sandbox).
5. It will have Multiplayer (Somebody hosts/joins).
You can do it with slick and kryoengine, take a look here for an example
6. It will have Online Mode (login and join to main server).
8. It will be P2P if possible in order to reduce server load.
You have a central server for authentication but a p2p architecture for your project? This huge architecture choices are based on how many players do you have. In my experience if you have a successfull project with 1000 players, you can afford a central server for your game (more true in a cloud system).
7. It will have Physics, such as collision between ships, gravity & etc.
This involves your gameplay: how will be? A space ship game with gravity require some sort of reality, not simulation, but this can affect your gameplay.
Basically if you will take Starfarer and EVE Online mix them, remove skill learning and level upping, add same module fitting system that was in good old X-COM 3 on ships, add multiple procedurally generated galaxies beside multiple procedurally generated star systems, you will get right direction of what I'm aiming to do.
Take a look to Drox Operative
My Java coding knowledge quite good. The problem is, that I never created any other Java programs beside console based ones, I never coded anything that had even slightest GUI, all programming languages, including java were used only to make console based applications.
Why I chooses Java? Because it's object oriented and it can be easily work on all possible PC OSes. I want to make game so it will be available for all OSes, that it could take advantage of 64-bit systems, SLIs, Crossfires, Multi-core/Multi-threading CPUs and in addition to everything have seamless network gameplay.
Creation of this game will be more like hobby for me, while I will continue with my standard work. I know, creation of such game could take couple of years. And I came to this forum to ask people with experience who know taste of failure and happiness of success for advice, where I need to start, where I need to begin, what Java/Slick libraries I need to use for creation of such game. I count on you and your advices.
My advise is to start small, then iterate:
1) build a top down game,
2) add skills
3) add multiplayer
4) add generated stuff
It's easy and you need motivations to do so!