how can i convert a c# class to a WSDL?

You have to write a WebService class. If you have a C# class with methods and properties, you will have to add [WebMethod], [WebService] etc attributes to your class to make it a WebService class.

You can build WSDL for a class by hand or by using Visual Studio. The simplest way is to create a Web Service project and add class methods and properties using VS.NET. To add all this by hand, you need to learn basics of Web Services and I am afraid there is no quick way to do so. You may want to dig MSDN for that.