As soon as your order is on its way you receive an email with your track & trace. That link lets you follow your parcel directly with PostNL.
No email received? Have a look in your spam folder first. You can also see the status of your order in your account.
Your track & trace may not show any information yet. That means your parcel has been registered but has not been scanned by PostNL. This is usually resolved within a day.