I'm making a multiplayer lobby based fighter game. I want there to be accounts and weapons bound to accounts.

how should I go about this?

Skills:

I can use Unity.

I know Ruby, Javascript, Python, C++ and Java

Hi, not much of a professional when it comes to this topic, but if I understand your goals correctly you are going to need a back-end / server-infrastructure.

For creating a client-server or a peer-to-peer game nothing "extra" is required, you can simply use the built-in networking api in Unity (as I know the new one is called Unet, but there is a legacy one if you use an older version). This will allow you creating a game where multiple players can play together over network.

For storing accounts, their data (like weapons/items bound to accounts), authenticating users and for registering dedicated servers or players hosting games, you are going to need a server machine and a back-end service/software. This is a whole different animal type than both the client application or the game server code/application, and I guess a lot of work if you don't really know what you are doing :(.

I would most probably go for the first goal at the beginning, and stack up knowledge about the second as I go, or find someone who is capable and can develop the second part, but if you insist on going forward, you should look into following technologies:

Database tech, SQL comes to mind (free SQL server: MySQL)
Back-end and server tech, maybe in your case this could be done using a web-server with a language like php (Ruby and Python is a common choice too), or you could develop a dedicated server application accessing your database using Java or C++.
You are going to need hosting too for your back-end, a virtual machine or a dedicated server machine or some kind of cloud solution.

Again, not proficient in this area, but as the general steps, something like this is correct...

For the lobby, you will need some kind of stable online server, with a name, that can be found.
The rest sounds like you can build it in top of the Unity networing API. You'd also need to use a database plugin of some sort for the Unity-based server, or use the WWW class to use a web-based database interface.
Can't really give more details without more specifics about what you have tried, what works, what doesn't, etc.

Thankyou everybody.. I understand :)

Btw: If you like writing Python code, a good way to get started on web services in Python is Flask: http://www.fullstackpython.com/flask.html