This series will get you up and running within UEFN so you can bring your ideas to life. It is aimed at those that are familiar with Fortnite Creative but are not familiar with Unreal Engine. Video Links Below.
(PLEASE NOTE: THERE HAVE BEEN SOME CHANGES TO THE UEFN SINCE THE CREATION OF THIS SERIES, PLEASE SEE THIS TUTORIAL FOR UPDATED INFORMATION ----------- UEFN for Fortnite Creators: Intro to UEFN --------------- Link/Video below 
https://dev.epicgames.com/community/learning/tutorials/j9K5/fortnite-intro-to-uefn