The First one is to get your free V-bucks card from the link below. then you have to redeem it at your game.
After you receive V-Bucks card, to Redeem a gift card for V-Bucks to use in Fortnite on any supported device! To use a gift card you must have a valid Epic Account, download Fortnite on a compatible device.
download Fortnite on a compatible device, and accept the applicable terms and user agreement. Review your device maker’s terms for any additional requirements to play Fortnite.
By deafult you can’t, but you can request more cards and redeem all of them