What exactly are you trying to do?
In VS.Net, you just need to create a Web Service Project in whatever language you are working in. From there, you'll probably want to change the .asmx file to something meaningful as well as change the Namespace from tempuri.org to your server or domain name.
From there, just add some Web-Methods to the class (by default, there's a hello world function you can play around with). Try to uncomment that, and compile the project. You should be able to set a web-reference to that project from another project (asp.net or win-forms, etc) and call the HelloWorld method and return a string to the caller.
Let me know if you want more detail or a specific scenario you're working on.