i have always wanted to make a private server but i don't know how i would do this. i know how a private server works, the game sends data packets to the server. the server will take the data and process it and send data to the other games connected.
my questions are,
the game will be something like WOW, i have not desided yet.
If you are hoping to embark on creating your own MMORPG then you have a huge task ahead of you, and unfortunately to put it nicely you are probably being too ambitious especially if you are asking these sorts of questions.
You should probably read up on client server architecture.
Also, in answer to your questions about the structure of the data being sent and how it is interpreted, well, that's 100% up to the people that design the system. You will want to simulate the entire game on the server(s) and don't trust the clients at all.
For something as complex as a MMORPG it is really important to create a solid design for the system before anything else, this is very important.
Just to be clear your intent is to create an emulated MMO server to the effect of WOW?
That's not really a trivial task and carries with it its own ethical implications.
Just to get started will require a ton of research, inspection, decoding, an extreme attention to detail.
If you are serious about it, then I would suggest looking up networking tools that can help you inspect traffic across the network and creating a scientific process for operation inspection.
Again, it should be noted this is by no means a trivial task.
This will be fairly difficult as you do not have the communicaton protocol specification for the game's client/server communication.
If you want to start this, then create a server that is simply a pass through. That is, all client requests are forwarded to the particular server. Once you have generated a large enough sample size of packets to study, then you can begin to dissect the meaning of each byte (possibly). Of course, if the packets are encrypted in any way (even a simple XOR encryption) then you will have an even harder time trying to figure out what each byte means. You should capture a sample set using two clients running sniffers so you can see what happens when one client does something and it needs to be sent to all clients.
But if I were you, I would just abandon the idea and work on something else. My two cents..
If you'd like an inside look at how games do networking, there's always Ryzom, which went open-source earlier this year. If you're creating your own MMO you can begin right there, and if you're looking to reverse-engineer one you can practice with your own client and server.