using  System.Data.SqlClient;

  public static string comSTR="Data Source= ......;Initial Catalog=Название базы ;User ID=имя ;Password=пароль";
  public  static string M;
      
  
        public Form_Export()
        {
            InitializeComponent();
            
        
            string connetionString = null;
            SqlConnection cnn;
            SqlCommand cmd;
            string sql = null;
            SqlDataReader reader;

            connetionString = @comSTR;
            sql = @"select  поле1  from t1 where  ваше условие ";

            cnn = new SqlConnection(connetionString);
            try
            {
                cnn.Open();
                cmd = new SqlCommand(sql, cnn);
                reader = cmd.ExecuteReader();

                while (reader.Read())
                {
                    M = Convert.ToString(reader["поле1"]);
                 }
                reader.Close();
                cmd.Dispose();
                 cnn.Close();
                
              }
            catch 
            {
                MessageBox.Show("Can not open connection !");
            }