How do i reduce the space taken up on my Mac? I need about 15 more GB to download Fortnite.

The files that you have control over are located in the Documents, Downloads, Pictures, Music and Movies folders.  You can use this free app, GrandPerspective, to find the largest files on your drive so you can determine if they can be deleted or moved to an external HD for storage.

Note: you can empty the Downloads folder after the apps and/or updates that were downloaded have been installed or applied.  Some users have found a couple of Gigabytes of .dmg files in their Downloads folder which are no longer needed. 

Also, for optimum system and app performance you should maintain a minimum of 80GB of free space on your boot drive.

That's the amount of space that the system takes up on the drive.

You'll have to go thru your Documents, Picture, Music, Movies and Applications folders and determine which files you can delete or move to an external drive to free up the space you need. You can have the folders show the size of the files/folders by going to each of those those folders and typing ⌘+J and checking the Size checkbox.

Free up storage space on your Mac - Apple Support