Paypal will send an email to the address you supplied when you opened the account.
You can also just go to your Paypal account and see whether the payment has arrived.
There is one occasional problem. If your buyer paid from his bank account (rather than from a PP balance or with a credit card) the payment will take about 10 days to clear. During this time, PP will not allow you to print a shipping label.*
Also, as a new seller, PP will not release the payment to you for 21 days, against your good performance. But you do have to ship within seven days of "cleared" payment (which means that PP has the money held in escrow for you).
*Using PP shipping labels not only keeps your books simpler, but some services have discounts between 5% and 18% off PO counter prices. Also you can drop your parcels in the nearest mailbox, skipping the PO entirely.
