You wait 21 days. 
New sellers and those who sell only occasionally have a 21-day hold placed on payments. Your paypal account will tell you when the payment has been cleared (e-cheques can take up to 10 days) and when it is okay to ship the item.
Once paypal tells you to ship the item, - do it. Don't wait. Here is the official ebay info page about those payment holds:
http://pages.ebay.ca/help/pay/payment_hold.html
