Web Service(Ajax) in asp.net

Hi,
I want to make a web service (Ajax) in asp.net.This web service should contain a method to login users and then the users are verified by database.
.i have taken many tutorials about Ajax.i know basic code steps in Ajax.I have worked in asp.net(web development) but i have no idea about Ajax web service.i am confused from where to start.i have installed Microsoft visual studio 2010.i know how to make default web service in asp.net but i don't know how to make Ajax web service and where i have to write code for login and how to connect this service with database.should i write code in service.cs file or service.asmx.or should an add another web form?Kindly help