Go to Phpmyadmin and run following sql code:
SET FOREIGN_KEY_CHECKS=0;
TRUNCATE `sales_flat_creditmemo`;
TRUNCATE `sales_flat_creditmemo_comment`;
TRUNCATE `sales_flat_creditmemo_grid`;
TRUNCATE `sales_flat_creditmemo_item`;
TRUNCATE `sales_flat_invoice`;
TRUNCATE `sales_flat_invoice_comment`;
TRUNCATE `sales_flat_invoice_grid`;
TRUNCATE `sales_flat_invoice_item`;
TRUNCATE `sales_flat_order`;
TRUNCATE `sales_flat_order_address`;
TRUNCATE `sales_flat_order_grid`;
TRUNCATE `sales_flat_order_item`;
TRUNCATE `sales_flat_order_payment`;
TRUNCATE `sales_flat_order_status_history`;
TRUNCATE `sales_flat_quote`;
TRUNCATE `sales_flat_quote_address`;
TRUNCATE `sales_flat_quote_address_item`;
TRUNCATE `sales_flat_quote_item`;
TRUNCATE `sales_flat_quote_item_option`;
TRUNCATE `sales_flat_quote_payment`;
TRUNCATE `sales_flat_quote_shipping_rate`;
TRUNCATE `sales_flat_shipment`;
TRUNCATE `sales_flat_shipment_comment`;
TRUNCATE `sales_flat_shipment_grid`;
TRUNCATE `sales_flat_shipment_item`;
TRUNCATE `sales_flat_shipment_track`;
TRUNCATE `sales_invoiced_aggregated`;
TRUNCATE `sales_invoiced_aggregated_order`;
TRUNCATE `sales_order_aggregated_created`;
TRUNCATE `sendfriend_log`;
TRUNCATE `tag`;
TRUNCATE `tag_relation`;
TRUNCATE `tag_summary`;
TRUNCATE `wishlist`;
TRUNCATE `log_quote`;
TRUNCATE `report_event`;
ALTER TABLE `sales_flat_creditmemo` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_creditmemo_comment` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_creditmemo_grid` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_creditmemo_item` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_invoice` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_invoice_comment` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_invoice_grid` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_invoice_item` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_order` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_order_address` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_order_grid` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_order_item` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_order_payment` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_order_status_history` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_quote` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_quote_address` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_quote_address_item` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_quote_item` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_quote_item_option` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_quote_payment` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_quote_shipping_rate` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_shipment` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_shipment_comment` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_shipment_grid` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_shipment_item` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_shipment_track` AUTO_INCREMENT=1;
ALTER TABLE `sales_invoiced_aggregated` AUTO_INCREMENT=1;
ALTER TABLE `sales_invoiced_aggregated_order` AUTO_INCREMENT=1;
ALTER TABLE `sales_order_aggregated_created` AUTO_INCREMENT=1;
ALTER TABLE `sendfriend_log` AUTO_INCREMENT=1;
ALTER TABLE `tag` AUTO_INCREMENT=1;
ALTER TABLE `tag_relation` AUTO_INCREMENT=1;
ALTER TABLE `tag_summary` AUTO_INCREMENT=1;
ALTER TABLE `wishlist` AUTO_INCREMENT=1;
ALTER TABLE `log_quote` AUTO_INCREMENT=1;
ALTER TABLE `report_event` AUTO_INCREMENT=1;
SET FOREIGN_KEY_CHECKS=1;
