You've been had by a phishing scam. These scams are so common anymore, that it's almost better to not put your information into anything at all. I can assure you that this has nothing to do with Best Buy. However, I can try to help you avoid this in the future. We'll deal with Best Buy, since this is the easiest example, and related to your issue...
Say you're browsing the web and you see an ad for a $1,000,000 gift card from Best Buy. You think, WOW that's an awesome deal! So, you click on the ad. Okay, that's alright, but stop before you do anything else. Look at the address bar. If it says anything other than http://www.bestbuy.com/ at the very start of the address, STOP! The site is a phony page set up for this reason: to get your information and bombard you. It's not set up to get you a gift card. There is no gift card.
Phishing scams are easy to set up...all the scammers setting up these scams is an image of the company, usually an official logo. With Best Buy, it's easy as the logo is pretty basic. Plus, you can do a google search and find just about any companies official logo. The scammers do is throw it onto the page to make it look more official, and then, once you put your information into the site, they sell your information to other companies, who continue to send you junk.
Hopefully this helps avoid scams like this in the future.