您需要GSM调制解调器(连接电缆和适配器(随附)和移动运营商提供的有效SIM卡.
然后，您需要了解AT命令，因为调制解调器可以理解AT命令.
然后，您需要了解使用任何.net语言(***使用C#)的串行端口通信.

而已！你完成了！您可以开始编写应用程序. :)
我建议您使用第3方库，这样您就不必对AT命令和串行通信有太多了解. GSMComm库就是这样一种广泛使用的库.
使用Google搜索有关它的更多信息.
您可以参阅本文如何使用GSM调制解调器发送和接收SMS [

You need a GSM modem (a connectivity cable and adapter - that comes with it) and a working SIM Card from a mobile operator.
Then you need to learn about AT commands because modems understand AT commands.
Then you need to know about Serial Port communication using any .net language (C# is preferable).

That''s it! you are done!! You can start writing your application. :)
I would suggest you to use 3rd party libraries so that you don''t need to know much about AT commands and Serial communication. One such widely used library is GSMComm Library.
Use google to search more about it.
You may refer this article How To Send and Receive SMS using GSM Modem[^]
This is a Windows application. But if you could understand it, writing Web Application would not be very difficult.

If you would have searched CodeProject (if not Google), you could have found similar question answered so many times. And there are lots of article available for the same. You may consider searching it now ;)
Hope this helps!