For Downloding Fortnite you want to download epic Game Laucher first
Once the Epic Games Launcher opens, find the page for "Fortnite." You'll be able to find it on the "Home" tab, or by searching for it in the "Store" tab.
If you have enough space on your computer, the game will begin downloading immediately. Wait for it to download — you can click "Downloads" in the left sidebar to see its progress — and once it's finished, launch it from the "Library" tab.