Once your code has arrived you can redeem your Best Buy gift card at the following locations:
You can redeem your gift card on location at Best Buy and online at Bestbuy.com
For an in-store purchase print out the gift card code and present it at the time of purchase.
For an online purchase, on the Checkout page select Gift Card and enter your gift card number.
Here is the illustration of the webpage.
You can check the balance here