how to get the list of all existing templates?

<?php

$modx->getObject('modTemplateVar', array('name'=>'MyTv'));  // if the tv exists already

// If not:
//$fields = array('name'=>'MyTv',
//    'type'=>'text',
//    'default_text=>'Some Text',
//    'caption'='My TV',
//    'description'='This is my TV'
// );
//$tv = $modx->newObject('modTemplateVar', $fields); 
//$tv->save();

$templates = $modx->getCollection('modTemplate');

foreach($templates as $template) {
    $intersect = $modx->newObject('modTemplateVarTemplate');
    $intersect->addOne($tv);
    $intersect->addOne($template);
    $intersect->save();
}

$tv->setValue($id,'value');  // where $id is the ID of a resource

$ids = $modx->getChildIds('0');  // will get an array containing all resources IDs.