If you want to create a Sharepoint web service, you can follow this link Walkthrough: Creating a Custom ASP.NET Web Service. When you done with creating, you might need to debug and trace it, here is how to do that.
- From Visual Studio, open your class library, then go to tools menu and choose attach to process
- Attach to process window will show, check show process from all users then scroll down to w3wp.exe -IIS worker process is a windows process (w3wp.exe) which runs Web applications-
You might have multiple running w3wp.exe, to determine which one to pick, go to IIS then open Worker Processes.
- You will find the processes id which runs your application's application pool.
- Go and choose the right w3wp.exe and know you can debug. :)