You start by figuring out what you actually want to make, rather than just a hodgepodge of bullet points.
Then you cry when you realize that what you just described is sort of like walking into NASA and saying "I want to make a rocket ship, how do I start?"
no.....
"how do I start?" more simple then that???
Some of the responses in this thread are less than helpful. 
Yes, it's probably a bit of a naive question, but instead of asking the OP for more info you just pull out the flamethrowers immediately? 
@OP - Could you give some more info on your programming experience and what games you have made before? Probably the way to go is going to be to learn some basic programming, to give a decent foundation (if you have not done so already), then move on to making a 2D game, then get a 3D engine, perhaps Unreal Engine 4 or Unity and start following tutorials on it.
3D networked games are really hard. It's a long long long hard road you are about to embark on, but take it one step at a time and maybe, just maybe, with a great deal of time and effort, you can get to the ability required. When it comes time for your final goal - A 3D multiplayer game, you will probably need very good knowledge of whatever engine you're using, the scripting within it and networking and you will still want to put together a team to get the project done with.
