Step 1:
If you’ve got a US Apple Account, head over to the App Store, search for Fortnite, then you can go and download the game!
If you don't have a US Apple Account, you can head over to the App Store, then switch to a US Apple Account, then download Fortnite using that account, or you can swap your current account country to the US, then download Fortnite, then swap back to your original country. If you need help with this, you can head over to step 2. Otherwise, enjoy Fortnite!