Umm i don't know about implementing A * algorithm on a server. You might run into performance issues. The algorithm can get really CPU expensive. Trying doing some research on that matter.
I would not recommend you implementing any complicated multiplayer interaction in the beginning. However if you must, take a look at this article, might give you some ideas. http://developer.valvesoftware.com/w...yer_Networking

When i say the server knows the players location, i mean that you have coded on the server the functionality that creates a player instance from a class, the socket listener for incoming requests and the parsing logic which does something with the request to the player class(or other classes.) And all of this code is running in one machine, the server.