You can create an observer that looks for the `sales_order_save_after` event and writes the xml out into a file using the order details from `getOrder()` and then ftp it over to somewhere that the ERP can see it.

I have a module that does something similar which might be a useful starting point. I have quickly replaced all code relevant to my site and have hopefully not missed anything in the find/replace. Create the files and just drop them into the directories below.

You need the following file structure:

 - app/local/Piyush/OrderExport/sql/piyush_orderexport_setup/install-1.0.0.php
 - app/local/Piyush/OrderExport/etc/config.xml
 - app/local/Piyush/OrderExport/Helper/Data.php
 - app/local/Piyush/OrderExport/Model/Observer.php
 - app/etc/modules/Piyush_OrderExport.xml

**install-1.0.0.php**

    <?php
    
    $installer = $this;
    $installer->startSetup();
    
    $installer->getConnection()
        ->addColumn($installer->getTable('sales/order'),'piyush_order_exported', array(
            'type'      => Varien_Db_Ddl_Table::TYPE_TEXT,
            'nullable'  => true,
            'length'    => 10,
            'after'     => null, // column name to insert new column after
            'comment'   => 'Is order exported'
        ));
    
    $installer->endSetup();

**config.xml**

    <?xml version="1.0" encoding="UTF-8"?>
    <config>
        <modules>
            <Piyush_OrderExport>
                <version>1.0.0</version>
            </Piyush_OrderExport>
        </modules>
        <global>
            <helpers>
                <Piyush_orderexport>
                    <class>Piyush_OrderExport_Helper</class>
                </Piyush_orderexport>
            </helpers>
            <models>
                <Piyush_orderexport>
                    <class>Piyush_OrderExport_Model</class>
                </Piyush_orderexport>
            </models>
            <resources>
                <Piyush_orderexport_setup>
                    <setup>
                        <module>Piyush_OrderExport</module>
                    </setup>
                </Piyush_orderexport_setup>
            </resources>
            <events>
                <sales_order_save_after>
                    <observers>
                        <Piyush_orderexport_order_save_after>
                            <type>singleton</type>
                            <class>Piyush_OrderExport_Model_Observer</class>
                            <method>exportOrder</method>
                        </Piyush_orderexport_order_save_after>
                    </observers>
                </sales_order_save_after>
            </events>
        </global>
    </config>

**Data.php**

    <?php
    
    class Piyush_OrderExport_Helper_Data extends Mage_Core_Helper_Abstract
    {
        const LOG_ENABLED_CONFIG_PATH = 'productimport/general_config/log_enabled';
        const LOG_FILE_NAME = 'Piyush_orderexport.log';
    
        /**
         * Utility function to log a message to the log file, if logging is enabled
         * @param string log message
         */
        public function log($message) {
            if (Mage::getStoreConfig(self::LOG_ENABLED_CONFIG_PATH)) {
                Mage::log($message, null, self::LOG_FILE_NAME);
            }
        }
    }

**Observer.php**

    <?php
    class Piyush_OrderExport_Model_Observer
    {
        const EXPORT_RELATIVE_PATH = 'var/export/orders/';
        const EXPORT_FILE_NAME = 'orderexport.xml';
        const FTP_HOST = "your.erp.ip.address";
        const FTP_PORT = 21;
        const FTP_USER = "";
        const FTP_PASSWORD = "";
    
        protected function sendFile($filePath, $fileName)
        {
            $_helper = Mage::helper('Piyush_orderexport');
            $_helper->log('Uploading the file to the FTP server...');
            if ($ftpResource = ftp_connect(self::FTP_HOST, self::FTP_PORT))
            {
                if (ftp_login($ftpResource, self::FTP_USER, self::FTP_PASSWORD))
                {
                    if (ftp_put($ftpResource, $fileName, $filePath, FTP_ASCII))
                    {
                        $_helper->log("Upload of remote file '$fileName' complete (source file is '$filePath').");
                    } else {
                        $_helper->log("Warning: error cannot upload the file!");
                    }
                } else
                {
                    $_helper->log("Warning: error cannot login to the FTP server! Can't upload the file.");
                }
            } else
            {
                $_helper->log("Warning: error connection to the FTP server! Can't upload the file.");
            }
        }
    
        public function exportOrder(Varien_Event_Observer $event=null)
        {
            $_helper = Mage::helper('Piyush_orderexport');
    
            $importStartDateTime = date('Y-m-d H:i:s');
            $_helper->log('*** EXPORT ORDER START - ' . $importStartDateTime . ' ***');
    
            /* @var Mage_Sales_Model_Order $order */
            $order = $event->getOrder();
            //$order = Mage::getModel('sales/order')->loadByIncrementId('100000042');
    
            $orderIncrementId   = $order->getIncrementId();
            $orderTimestamp     = strtotime($order->getCreatedAt());
            $orderDate          = date("ymd", $orderTimestamp);
            $orderTime          = date("Hi", $orderTimestamp);
            $orderShipping      = number_format(round($order->getShippingInclTax(), 2),2);
    
            $_helper->log('Processing order '.$orderIncrementId);
            $_helper->log('Order state = '.$order->getState());
            $_helper->log('Order has been exported is '.$order->getPiyushOrderExported());
    
    
    	   if ($order->getPiyushOrderExported() != '1' && $order->getState() == $order::STATE_NEW) {
    
                $_helper->log('Exporting order '.$orderIncrementId);
                $_helper->log("\t- date = $orderDate");
                $_helper->log("\t- time = $orderTime");
                $_helper->log("\t- shipping = $orderShipping");
    
                $xmlFilePath = Mage::getBaseDir('base') .'/'. self::EXPORT_RELATIVE_PATH . self::EXPORT_FILE_NAME;
                $outputFileHandler = fopen($xmlFilePath,"w");
    
                $orderItems = $order->getAllVisibleItems();
                foreach ($orderItems as $item)
                {
                    //$_helper->log("Item data: ".print_r($item->getData(),true));
    
                    $itemSku                = $item->getSku();
                    $itemQty                = number_format(round($item->getQtyOrdered(), 2),2);
                    $itemRowTotal           = number_format(round($item->getRowTotal(), 2),2,'.','');
                    $itemRowTotalInclTax    = number_format(round($item->getRowTotalInclTax(), 2),2,'.','');
                    $itemTaxAmount          = number_format(round($item->getTaxAmount(), 2),2,'.','');
                    $itemPlu                = Mage::getModel('catalog/product')->loadByAttribute('sku', $itemSku)->PiyushPlu();
    
                    $_helper->log("\t- processing order item $itemSku:");
                    $_helper->log("\t\t- qty = $itemQty");
                    $_helper->log("\t\t- row total = $itemRowTotal");
                    $_helper->log("\t\t- tow total incl tax = $itemRowTotalInclTax");
                    $_helper->log("\t\t- tax amount = $itemTaxAmount");
                    $_helper->log("\t\t- PLU = $itemPlu");
                    
    				$xmlString="";
                    /* create your xml here */
                    
                    fputs($outputFileHandler,$xmlString);
                    //$_helper->log("xmlString = $xmlString");
                    
                }
    
                fclose($outputFileHandler);
    
                $this->sendFile($xmlFilePath, self::EXPORT_FILE_NAME);
    
                // $order->setGcmOrderExported('1'); Typo corrected
                $order->setPiyushOrderExported('1');
                $order->save();
    
                $importEndDateTime = date('Y-m-d H:i:s');
                $_helper->log('*** EXPORT ORDER END - ' . $importEndDateTime . ' ***'.PHP_EOL.PHP_EOL);
            }
        }
    }

**Piyush_OrderExport.xml**

    <?xml version="1.0"?>
    <config>
        <modules>
            <Piyush_OrderExport>
                <active>true</active>
                <codePool>local</codePool>
            </Piyush_OrderExport>
        </modules>
    </config>