Hi there
You would need to empty all tables that start with orders_* using phpmyadmin or similar. That should then restart your orders at "1". However doing this it would also be best to remove all of your test customers again empty all tables starting with customers_* and the table address_book. Note that yoor tables might also have a prefix of zen_. And before your start make a backup you can do this from your cpanel or you can install a contribution from zen-cart.com for this.