void FillLogins(string userName, string password)
    // Open connection
    using (SqlConnection c = new SqlConnection(yourConnectionString))
    {
    c.Open();
    // Create new DataAdapter / Command
    using (SqlDataAdapter a = new SqlDataAdapter(
        "Select Count (*) From dbo.[LOGIN] where Username= @userName AND Password = @password", c))
      {
        a.SelectCommand.Parameters.AddWithValue("@userName", userName);
        a.SelectCommand.Parameters.AddWithValue("@password", password);
        // Use DataAdapter to fill DataTable
        DataTable t = new DataTable();
        a.Fill(t);
      }
   }
}

FillLogins(USERNAME_txt.Text, password_txt.Text);