data management system with server and client

You didn't specify much requirements so I assume that you currently have no exact spec. Here's few thoughts:

From the technology point of view, you can use ODP.NET for the communication between the client and the database. What comes to user interface, consider if you want to create a client-server application or do you want to run the software in a browser.

If you're going to use client server, in my opinion WPF would be a good technology. But if you want to use browser, then you must have separate layers for the logic and the ui. With C# you can use IIS for the middle tier and MVC 3 along that.

If you haven't got or defined the requirements yet, that's the place to start. Get a clear understanding what is needed and expected and document it well. Then you can consider how much logic you want to put in the client application and how much you're going to utilize the capabilities of the database server. By using the database server I mean creating packages with PL/SQL, using triggers and so on. Either way I strongly suggest using declarative constraints (foreign keys, checks...) quite heavily at database side.

During the process, advancing by small steps and prototyping is always a good choice. Create a small piece of the program, add the necessary logic, build the UI and test how it works and how understandable the solution feels like. This helps to correct the course as early as possible if needed.

You can start by....
http://msdn.microsoft.com/en-us/library/system.net.sockets.socket.aspx[^]
http://msdn.microsoft.com/en-us/library/kwdt6w2k(v=VS.71).aspx[^]