When your order was shipped, you should have received a Shipping Confirmation email from firstname.lastname@example.org with your tracking number in it. A quick search for this email address in your email account will be the easiest way to locate it.
However, if you still can’t find it, the next thing to do is check your spam folder and any filters you may have created for organising emails. More often than not this is where they hide away.
You’ll then be able to track your order via the USPS website.