Please help me program to monitor server in C#

This is a big question. Let's do one step at a time.

Let's start with memory usage:

To monitor memory usage, you can use System.Diagnostics. Please see how to monitor memory per-process:
http://msdn.microsoft.com/en-us/library/s80a75e5%28v=vs.90%29.aspx[^].

(Sorry, the article is only for v.3.5 and below, but the methods themselves are not obsolete. Please check the MSDN help on each method referenced in this article to find the version for .NET Framework version you nee.)

You can summarize the memory usage by all processes you are interested in. You can get the an array of processes using the class System.Diagnostics.Process and one of its methods GetProcesses or GetProcessesByName, please see:
http://msdn.microsoft.com/en-us/library/system.diagnostics.process.aspx[^].

—SA

Now, let's see how to monitor hard drives.

You can use the class System.IO.DriveInfo, please see http://msdn.microsoft.com/en-us/library/system.io.driveinfo.aspx[^].

DriveInfo[] drives = DriveInfo.GetDrives();
foreach (DriveInfo drive in drives) {
   if (drive.IsReady) {
       long freeSpace = drive.TotalFreeSpace;
       long size = drive.TotalSize;
       string name = drive.Name;
       //see the MSDN help page reference info for other drive properties
       //...
       ReportDriveInfo(freeSpace, size, name /* , ... */); //whatever you need for monotoring
   } else
      // do something to deal with non-ready drive; normally, it should not happen
}

Also, you can use push technology (http://en.wikipedia.org/wiki/Push_technology[^]) in monitoring drives. Instead of polling data periodically, which is always not very good, you can subscribe to some file system events and avoid polling data if nothing essential has changed. For this purpose, you can use the class System.IO.FileSystemWatcher; please see:
http://msdn.microsoft.com/en-us/library/system.io.filesystemwatcher.aspx[^].

You may find the idea of push technology very useful if you remember that file operations are relatively expensive, so you want to reduce number of polls.

—SA

[Answering a follow-up question: how to monitor network traffic usage]

This is just one of the approaches: Microsoft provides a tool to monitor it, called Network Monitor:
http://en.wikipedia.org/wiki/Network_monitor[^],
http://blogs.technet.com/b/netmon/archive/2008/09/17/network-monitor-3-2-has-arrived.aspx[^],
http://www.microsoft.com/openspecifications/en/us/applied-interoperability/testing/network-monitor/default.aspx[^].

Download:
http://www.microsoft.com/download/en/details.aspx?displaylang=en&id=4865[^].

It can be uses as is, but if you want to develop some monitoring code, it provides the API. Please see:
http://blogs.technet.com/b/netmon/archive/2008/10/29/intro-to-the-network-monitor-api.aspx[^].

I personally did not use it.

Another piece of software is offered by this CodeProject article:
IP Helper API - Retrieve TCP/IP/UDP Statistics, Interface details, ARP Table and Route Table[^].

Unfortunately, this is not written in .NET. You will need to use native DLL in your C# project. You have two options:

• Make your C++ project an unmanaged DLL. Use P/Invoke to consume your DLL in a .NET project. Please see:
  
  http://en.wikipedia.org/wiki/PInvoke[^];
  http://msdn.microsoft.com/en-us/library/Aa712982[^].
  
  This CodeProject article could also be useful:
  http://www.codeproject.com/csharp/EssentialPInvoke.asp[^].
• Develop a mixed mode C++/CLI + C++ (managed+unmanaged) DLL out of your C++ application. Wrap you C++ code in managed ("ref") C++/CLI classes and make them and required method public. Resulting DLL can be used as a regular .NET assembly: referenced in your .NET project.
  
  Please see:
  http://en.wikipedia.org/wiki/C%2B%2B/CLI[^],
  http://www.ecma-international.org/publications/standards/Ecma-372.htm[^],
  http://msdn.microsoft.com/en-us/library/xey702bw.aspx[^].

—SA

Form monitoring system memory, you can use the class System.Diagnostics.PerformanceCounter, please see:
http://msdn.microsoft.com/en-us/library/system.diagnostics.performancecounter%28v=vs.100%29.aspx[^].

I think this simple code sample can give you clear idea on how to use it for CPU usage monitoring:
http://stackoverflow.com/questions/278071/how-to-get-the-cpu-usage-in-c[^].

—SA

PerfMon Sample: Demonstrates How to Monitor System Performance Using Performance Counters[^] Is something that I would start to read and understand.