If you do not have your USPS tracking number then there's not too much you can do other than look in your email account for an email that may contain your tracking number if you've ordered something.
Or contact the seller or shipper of the package and see if they can find the tracking number for whatever it is you purchased.
Sometimes I've had to contact the shipper of the package and ask them to find and send me the tracking number again and they gladly sent me the lost tracking number.
Also look into your order history of the website you ordered the item from and the tracking number may be in there as well.