how to implement payment gate way in asp.net

tell me how it works

You need to finalize which option you want to use there are a lot of option are availble for example

paypal
authorize.net
google checkout etc.

Based on then you can get the API for each of the same to configure in your application.

here is a sample code for Paypal

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using com.paypal.sdk.profiles;
using com.paypal.sdk.services;
using com.paypal.sdk.services;
using log4net;
public partial class PaypalPayment : System.Web.UI.Page
{

protected void Page_Load(object sender, EventArgs e)
{
CallerServices caller = new CallerServices();
IAPIProfile profile = ProfileFactory.CreateAPIProfile();
profile.APIUsername =“nitin._1214308341_biz_api1.gmail.com”;// “sdk-seller_api1.sdk.com”;
profile.APIPassword =“1214308348”;// “12345678”;
profile.Environment =“sandbox”;
profile.Subject ="";
profile.APISignature =“A5GwH2U8prt9wuPnGmlizZVpeMthAn39AxoXc.I0NyrkrNlL8dUfnjII”;
caller.APIProfile = profile;
com.paypal.soap.api.DoDirectPaymentRequestDetailsType directPaymentDetails = new com.paypal.soap.api.DoDirectPaymentRequestDetailsType();

//Set Credit Card
com.paypal.soap.api.CreditCardDetailsType cc = new com.paypal.soap.api.CreditCardDetailsType();
cc.CreditCardType = com.paypal.soap.api.CreditCardTypeType.Visa;
cc.CreditCardNumber =“4111111111111111”;
cc.ExpMonth=10;
cc.ExpYear = 2009;
cc.CVV2 =“123”;
//Set Credit Card.CardOwner
com.paypal.soap.api.PayerInfoType theCardOwner = new com.paypal.soap.api.PayerInfoType();
com.paypal.soap.api.PersonNameType thePayerName = new com.paypal.soap.api.PersonNameType();
thePayerName.FirstName =“John”;
thePayerName.LastName =“Doe”;
theCardOwner.PayerName = thePayerName;
com.paypal.soap.api.AddressType theAddress = new com.paypal.soap.api.AddressType();
theAddress.Street1 =“123 Main”;
theAddress.Street2 =“Apt 23”;
theAddress.CityName =“Hill Side”;
theAddress.StateOrProvince =“FL”;
theAddress.PostalCode =“32550”;
theAddress.Country = com.paypal.soap.api.CountryCodeType.US;
theCardOwner.Address = theAddress;
cc.CardOwner = theCardOwner;
directPaymentDetails.CreditCard = cc;
//Set Order Total
com.paypal.soap.api.BasicAmountType temp = new com.paypal.soap.api.BasicAmountType();
com.paypal.soap.api.PaymentDetailsType payDetailType = new com.paypal.soap.api.PaymentDetailsType();
temp.Value =“118.25”;
temp.currencyID = com.paypal.soap.api.CurrencyCodeType.USD;payDetailType.OrderTotal = temp;
directPaymentDetails.PaymentDetails = payDetailType;
//Set IP
directPaymentDetails.IPAddress =“192.168.200.195”;
//Set Transaction Type
directPaymentDetails.PaymentAction = com.paypal.soap.api.PaymentActionCodeType.Sale;

//Set Request
com.paypal.soap.api.DoDirectPaymentRequestType request = new com.paypal.soap.api.DoDirectPaymentRequestType();

//Set Payment Detail
request.DoDirectPaymentRequestDetails = directPaymentDetails;

request.DoDirectPaymentRequestDetails.CreditCard.CardOwner.Address.CountrySpecified =true;
com.paypal.soap.api.DoDirectPaymentResponseType response = (com.paypal.soap.api.DoDirectPaymentResponseType)caller.Call(“DoDirectPayment”
, request);
Response.Write(“response=” + response.Ack);
Response.Write(
"
response=" + response.Errors[0].LongMessage.ToString() + " - " + response.Errors[0].ErrorCode.ToString());

//