If you have your coins they are in a bitcoin wallet (like blockchain.info's) or an exchange account (like coinbase). Either way you will see a button for "send bitcoins" click on it and it asks for an address.
When you buy an item on little biggy you get a bitcoin address to pay for it. You get the address when you click on the "pay now" button. Just copy and paste it and you're done. You usually see confirmation in a minute.
Relax and remember if you have any problem there is no way to "lose" bitcoins, the record of where the coins went is public. There is no " I sent the check in the mail "nor " I never got your payment.