ASP.NET

MailMessage mailmsg= new MailMessage();
  mailmsg.From = new MailAddress("Your Email Address");
  mailmsg.To.Add("Email of reciver");
  mailmsg.Subject = "Your sybject";
  mailmsg.Body = "The body of your mail (You can use html tags too";
  mailmsg.IsBodyHtml = true;
  SmtpClient smpt= new SmtpClient("127.0.0.1"); [if you are using your application in your own host]
  smtp.Send(mailmsg);

string subject = "Your Subject";
        string from = "Your Gmail account having @gmail.com";
        string rec = "reciver email";
        string body = "The mail body.";
        MailMessage mail = new MailMessage(from, rec);
        mail.IsBodyHtml = true;
        mail.SubjectEncoding = System.Text.UnicodeEncoding.UTF8;
        mail.Subject = subject;
        mail.Body = body;
        SmtpClient sm = new SmtpClient();
        System.Net.NetworkCredential me = new System.Net.NetworkCredential();
        me.UserName = "your gmail username";
        me.Password = "your gmail password";
        sm.UseDefaultCredentials = false;
        sm.Credentials = me;
        sm.Host = "smtp.gmail.com";
        sm.Port = 587;
        sm.EnableSsl = true;
        sm.Send(mail);

try
{
  ... try to insert new record ...
}
catch(SqlException se)
{
  ... set error message to something appropriate ...
}

Dim strQuery as string = "" 
strQuery should be where you select * from users where username = username you're trying to test
Dim conn As New SqlConnection(ConfigurationManager.ConnectionStrings("mainDBConnection").ConnectionString)
conn.Open()
Dim cmd As New SqlCommand(conn.Database & ".dbo." & strQuery, conn)
Dim dr As SqlDataReader = cmd.ExecuteReader(CommandBehavior.CloseConnection)
if dr.HasRows = true then
'DISPLAY ERROR MSG
end if