database inittialization for a plugin

hello folks, i am almost new to the vast PHP programming world , and i have some concepts that i misundestand…

here is my code

<?php
recojerfechas();
$action = checkGet('action','');

switch($action)
    {

        case "pintarfechas":
            recojerfechas();
        break;
    }

function recojerfechas ()
{
    global $wpdb;
    $table_name     = $wpdb -> prefix ."temporada" ;
    $query          = "SELECT * FROM $table_name;";
    $result         =  $wpdb -> query($query);
        while ($datostemporada = mysql_fetch_row($result))
                {
                    $idfila = $datostemporada[0];
                    global $wpdb;
                    $table_name = $wpdb -> prefix ."tarifas";
                    $sql= " SELECT * FROM $table_name WHERE id_tarifa = $datostemporada[4]";
                    $resulta    = mysql_query($sql);
                    $datostarifaaplicada = mysql_fetch_row($resulta);
                echo "[$datostemporada[2],$datostemporada[3],$datostarifaaplicada[3]]";
                }
}
?>

and this are my errors
Notice: Trying to get property of non-object in /usr/local/pem/vhosts/118777/webspace/httpdocs/wp-content/plugins/reservations/pintor_de_rangos.php on line 16 Fatal error: Call to a member function query() on a non-object in /usr/local/pem/vhosts/118777/webspace/httpdocs/wp-content/plugins/reservations/pintor_de_rangos.php on line 18

Yes, i am calling this file directly from my Navigator… .You all might say that — What is doing a guy that doesnt know how to instanciate an object , making a plugin ? well i am trying to learn….
What a heck is an object , why at my main plugin page works and not here…. i wanted to create more than one page, please dont refer me to the codex i come from there , i even copy pasted examples but i always get the same error , it seems i need another file (include something) or that i need to instaciate an object , which honestly i dont know what it means…
Thanks for reading till here.