If they are very new, you could cut them some slack and offer to cancel the transaction, but normally, you would wait 4 days and then open an unpaid item dispute by using the drop down tabs at the right of the item, resolve a problem. Follow the prompts to close when able and you will get your final value fees back and the buyer will be issued a strike.
Add them to your blocked bidder list so they cant do this to you again, access that from the site map, bottom of any page.
If you do decide to offer them a cancellation, make sure that they will agree to it first, before sending one from the same place you open a non payer dispute.
