There are many ways to integrate with Authorize.NET. One of the simpler ways is to post to the Authoerize.NET site and get a response. Doing so requires populating the post with all of the fields Authorize.NET is expecting. Here is some sample code that does a post and gets back a response:

/// <summary>
/// This function will authorize the payment by posting directly to Authorize.NET
/// </summary>
public void AuthorizePayment()
{
string AuthNetVersion = "3.1"; // Contains CCV support
string AuthNetLoginID = ConfigurationManager.AppSettings["Authorize.NETLoginID"].ToString();
string AuthNetPassword = ConfigurationManager.AppSettings["Authorize.NETPassword"].ToString();
// Get this from your authorize.net merchant interface
string AuthNetTransKey = ConfigurationManager.AppSettings["Authorize.NETTransKey"].ToString();

Boolean blnTestTransaction = Convert.ToBoolean(ConfigurationManager.AppSettings["Authorize.NETTestTransaction"].ToString());

WebClient objRequest = new WebClient();
System.Collections.Specialized.NameValueCollection objInf =
new System.Collections.Specialized.NameValueCollection(30);
System.Collections.Specialized.NameValueCollection objRetInf =
new System.Collections.Specialized.NameValueCollection(30);
byte[] objRetBytes;
string[] objRetVals;
string strError;

objInf.Add("x_version", AuthNetVersion);
objInf.Add("x_delim_data", "True");
objInf.Add("x_login", AuthNetLoginID);
objInf.Add("x_password", AuthNetPassword);
objInf.Add("x_tran_key", AuthNetTransKey);
objInf.Add("x_relay_response", "False");

objInf.Add("x_test_request", blnTestTransaction.ToString());

objInf.Add("x_delim_char", ",");
objInf.Add("x_encap_char", "|");

// Billing Address
objInf.Add("x_first_name", this.CustomerFirstName);
objInf.Add("x_last_name", this.CustomerLastName);
objInf.Add("x_address", this.CustomerAddress);
objInf.Add("x_city", this.CustomerCity);
objInf.Add("x_state", this.CustomerState);
objInf.Add("x_zip", this.CustomerZIP);
objInf.Add("x_country", this.CustomerCountry);
objInf.Add("x_phone", this.CustomerPhone);

objInf.Add("x_description", this.Description);
objInf.Add("x_po_num", this.PONumber);

// Card Details
objInf.Add("x_card_num", this.CCNumber);
objInf.Add("x_exp_date", this.ExpirationDate);

// Authorisation code of the card (CCV)
objInf.Add("x_card_code", this.CVV2Number);

objInf.Add("x_method", "CC");
objInf.Add("x_type", "AUTH_CAPTURE");
objInf.Add("x_amount", this.Amount.ToString());

// Currency setting. Check the guide for other supported currencies
objInf.Add("x_currency_code", "USD");

try
{
// Pure Test Server
objRequest.BaseAddress =
"https://secure.authorize.net/gateway/transact.dll";

objRetBytes =
objRequest.UploadValues(objRequest.BaseAddress, "POST", objInf);
objRetVals =
System.Text.Encoding.ASCII.GetString(objRetBytes).Split(",".ToCharArray());

this.ResponseCode = objRetVals[0].Trim(char.Parse("|"));

if (objRetVals[0].Trim(char.Parse("|")) == "1")
{
// Returned Authorization Code
//this.ResponseCode = objRetVals[4].Trim(char.Parse("|")); ;
// Returned Transaction ID
this.TransactionID = objRetVals[6].Trim(char.Parse("|"));
this.ResponseMessage = "Approved";
}
else
{
// Error!
strError = objRetVals[3].Trim(char.Parse("|")) + " (" +
objRetVals[2].Trim(char.Parse("|")) + ")";

if (objRetVals[2].Trim(char.Parse("|")) == "44")
{
// CCV transaction decline
strError += "Our Card Code Verification (CCV) returned " +
"the following error: ";

switch (objRetVals[38].Trim(char.Parse("|")))
{
case "N":
strError += "Card Code does not match.";
break;
case "P":
strError += "Card Code was not processed.";
break;
case "S":
strError += "Card Code should be on card but was not indicated.";
break;
case "U":
strError += "Issuer was not certified for Card Code.";
break;
}
}

if (objRetVals[2].Trim(char.Parse("|")) == "45")
{
if (strError.Length > 1)
strError += "<br />n";

// AVS transaction decline
strError += "Our Address Verification System (AVS) " +
"returned the following error: ";

switch (objRetVals[5].Trim(char.Parse("|")))
{
case "A":
strError += " the zip code entered does not match " +
"the billing address.";
break;
case "B":
strError += " no information was provided for the AVS check.";
break;
case "E":
strError += " a general error occurred in the AVS system.";
break;
case "G":
strError += " the credit card was issued by a non-US bank.";
break;
case "N":
strError += " neither the entered street address nor zip " +
"code matches the billing address.";
break;
case "P":
strError += " AVS is not applicable for this transaction.";
break;
case "R":
strError += " please retry the transaction; the AVS system " +
"was unavailable or timed out.";
break;
case "S":
strError += " the AVS service is not supported by your " +
"credit card issuer.";
break;
case "U":
strError += " address information is unavailable for the " +
"credit card.";
break;
case "W":
strError += " the 9 digit zip code matches, but the " +
"street address does not.";
break;
case "Z":
strError += " the zip code matches, but the address does not.";
break;
}
}

// strError contains the actual error
this.ResponseMessage = strError;
}
}
catch (Exception ex)
{
this.ResponseMessage = ex.Message;
}
}

Best of luck!

-Bryan Soltis