string queryString = "SELECT SUM(skupaj_kalorij)as Skupaj_Kalorij  "
                + "FROM (obroki_save LEFT JOIN users ON obroki_save.ID_uporabnika=users.ID)"
                + "WHERE (users.ID= " + a.ToString() + ") AND (obroki_save.datum= @datum)";

            using (OleDbCommand cmd = new OleDbCommand(queryString,database))                                    
                {
                    DateTime datum = DateTime.Today;
                    cmd.Parameters.AddWithValue("@datum", datum);
                }
            loadDataGrid2(queryString);

I tried now with parameters. But i don't really know how to do it correctly. I tried like this, but the parameter datum doesn't get any value(according to c#).