Hello 'christina',
If you have won an auction which you wish to cancel, the only way to do that is to contact the seller. You must very nicely ask for a cancellation. If the seller agrees, he will send you a Request for Cancellation, which you will quickly agree to accept. The seller has to pay fees on items that are 'sold', and he will want his fees back.
If the seller does not wish to cancel the sale, he can file an Unpaid Item dispute against you, at which point you must either pay for the item or get a strike on your record. If you get a second strike, you will find you are automatically blocked from just about every seller on ebay for the next year. Auctions are loads of fun, but when you win you are expected to pay. You don't have to want to, you just have to do it. Or get a strike.
By now I'm sure I don't need to stress the importance of carefully reading every word of a listing. Taking a moment to read the whole thing can save headaches later. Or avoid strikes, at any rate. Have a look at the following link that tells about Unpaid Items:
http://pages.ebay.ca/help/buy/questions/unpaid-item.html
Best of luck. 🙂