First: Thanks for being deployed. If you can't download things from certain sites (bandwidth limitation, or filter?), once you come to a definite conclusion, you could decide what downloads you want, and ask a family member to download them onto a cheap flashdrive (it'll all fit into a 4GB drive, $10) and mail the flashdrive to you, if this doesn't violate your base's security rules (ask your superiors!).
Second: If your goal is a RPG that runs in a browser, but your first goal is learning how to program, you'd probably want a language that can help with both those goals.
C++
cannot run in the browser. It
can be used for the server-side, but
not the client-side,
unless the client is on the user's PC (like Everquest) and not in a webpage (like Runescape).
C++: Can run the server,
can run a downloadable client,
can't run a webpage client
Python: Can run the server,
can run a downloadable client,
can't run a webpage client
Java: Can run the server,
can run a downloadable client,
can run a webpage client
Flash: Can't run the server,
can sort-of run a downloadable client (with some work-arounds),
can run a webpage client
Javascript: Can't run the server,
can't run a downloadable client,
can run a webpage client
It's not unusual for the server to be coded in a different language than the client - but if you are a beginner starting out, this does add complications for you.
Perhaps others can offer additional suggestions, or correct any mistake I might've made in the above - My experience is primarily C++ with only a little Python and very little Javascript, so I might've gotten some details wrong.
C++: Hard-ish to learn, but still definitely doable. More "closer to the metal", requiring more code to do less, and making for slower development time, but gaining faster speeds.
Python: Easier to learn. Runs fast, but not as close to the metal which means faster development time (less code to do more).
Java: My impression is that it is somewhere between C++ and Python in terms of difficulty and development time,
but I've never used Java, so this is speculation.
Your game's goals:- 2D, online (100 person community, 25 or less on at once), RPG, in a webpage.
Your game's requirements:- Client speed isn't an issue (it's 2D).
- Server speed isn't an issue (it's 25 or less people).
- Client needs to run in a webpage.
Using the knowledge above, you choose the right tool for the right job, from multiple options available, while keeping in mind both your short-term (Learning programming through Pong or Tetris or another small game or two) and long-term (Small online RPG) goals.
If you are set on your game running within a webpage, it would seem to me,
as someone who never made webpage games, that Java might be the best choice.
If you are willing to have your game run on the user's PC instead of in a webpage, I'd recommend
Python, but C++ would also work (it'll just take longer).