The most straightforward way to unlock free Fortnite V-Bucks is to level up with experience points through completing Fortnite quests. As you level up you’ll reach new tiers in the Battle Pass system (both the free and paid version) and a handful of these tiers will give you 100 V-Bucks each time to spend in the Fortnite store.
Levelling up comes best from playing aggressively and getting kills, surviving longer, and getting an experience bonus. All these accomplishments will give you experience points that level you up, so it pays to play more and that’s exactly what Epic wants you to do.