When purchasing an item, you need to put in a refund address. You will see a screen like this prompting you to enter your own Bitcoin address.
A refund address is a Bitcoin address which you control, and is only used in case you are issued a refund on your purchase. You cannot continue with the purchase unless you enter a valid Bitcoin address because you would be unable to receive a refund if needed.
If you don't have a Bitcoin wallet there are many available to choose from. You can read our post on Bitcoin wallets here [soon to come]. An example of a simple online wallet to use is RushWallet - don't forget to bookmark the page if you use their service.