I want to make a chat server by using .NET technology (C# and SQL server) and chat clients with iOS Apps and C# Apps but i don't know where to begin :( Anybody can tell me what i need to know to do it :(
Making chat server with .NET and Objective-C in iOS?
What you are trying to do is perfectly possible.
If you are doing it as homework of for the sake of learning, I would suggest start learning some general Network Programming concepts (sockets, TCP/IP, protocols). After that, you will need to learn of to do Network Programming in .NET and in C (or Objective-C). After that, you will need to design and implement a CHAT protocol (unless you are planning to use an existing one like IRC). Then, jump into coding the server in C# or VB.NET, and the client in C, C or Objective-C.
If you are trying to develop a product, try reusing as much components as you can. For example, use Jabber or IRC as your communication protocols. You will find several libraries you can use in C#, Java, Objective-C and C to implement the software you are planning to.