You have to press three buttons to complete the sale, so if you only pressed it once, don't worry.
If you actually pressed more than once, you can use Contack Seller on the listing to explain and apologize. The seller then has two choices.
She can send you a Mutual Cancellation. Agree to this immediately. This will end the transaction and give her the fees she paid eBay for the sale back. Leave positive feedback like "Thank you for helping a newbie."
Or she can start an Unpaid Item Dispute. If you do not pay when you get this, you will have a Strike against you which will make it more difficult to bid in future with thousands upon thousands of eBay sellers who Block bidders with Strikes. You will not be able to leave feedback.
