Networking,make the hole game myselfe?
That being said: The amount of work is such that most people don't get there.
My recommendation would be to either design a very simple 2D game environment (like agar.io or Realm of the Mad God) or use a pre-existing game engine with pre-existing 3D art, because learning to create art is just as time-consuming (if not more) as learning programming.
One thing you could look at is Unreal Engine 4. It's now free to download, it has a marketplace where you can buy 2D and 3D graphics parts (player characters, weapons, trees, etc) and it has a "blueprint" scripting environment that is a gentle introduction to creating gameplay code without having to learn C++ or C# right away.
Unreal Engine 4 also has built-in networking, that would be able to support the 64-player limit. It also supports larger, streaming worlds that let you build the "open world" you want.
If you sell the game, you'll have to pay Epic 5% of your revenues as royalties. Totally worth it for getting all of that game engine without having to write it yourself :-) (and, seriously, that engine represents thousands of man-years of effort!)
A single person can almost make any game given enough experience and time. With that said, don't expect your dream to be done anytime soon, it's going to take a lot of time and dedication to get there. Questions like whether you'll need a server or not are all pointless at this point in time, you need to be focusing on getting the experience to create games first. The beginner's forum is an excellent place to start.
I think that your best choice is to learn something like Unity3D it has a lot of tutorials and networking after the 5.1 release it's pretty easy.
Here there are some interesting links:
Unity Tutorials: (I suggest you to begin from here)
http://unity3d.com/learn/tutorials
Unity Docs:
http://docs.unity3d.com/Manual/index.html
Networking with Unity3d:
http://docs.unity3d.com/Manual/UNet.html
Good luck ;)
Well, not every networked game needs a server... us old farts around here remember a time when most games that where "online" actually where peer-to-peer games, either played in the LAN or later with special Matchmaking servers.
Which of course comes with a whole host of its own problems, first of all the increased ability of players to cheat. But, if you accept that (and even client-server games today have to accept to some extent that even with the best security measures, cheating is still existing), you can cut out the biggest expense in an online game, or at least reduce its cost (if the server is only a "chat lobby" with matchmaking, you can probably host 10-100x as many players on the same hardware).
Will not be much simpler to develop though (you can cut out the whole server simulation / client synchronization part if you do it really peer to peer, the different clients still need to be synchronized... and many games still had a server, just one of the clients would start a server on one of the client machines).
