authorize.net

I want to implement Authorize .net payment gateway in my website using asp.net. I am a beginner in this. Can someone give me a sample code from where I can be redirected to the Authorize.net page to complete payment process. I have created a sandbox account. Redirection URL - https://test.authorize.net/gateway/transact.dll but I get an error (13) The merchant login ID or password is invalid or the account is inactive. My account is active and in test mode. My Code: protected void Button_pay_Click(object sender, EventArgs e) { string value = TextBox_amt.Text; decimal d = decimal.Parse(value);