I thought my files were being backed up into iCloud.

+1

You should create a Time Machine backup. To start, you need an external hard drive. Here’s a few excellent ones sold by Apple: 

• LaCie Mobile Drive 4TB External Hard Drive USB-C - Apple
• LaCie 4TB Rugged USB-C Portable Hard Drive - Apple

If you have an iMac (Late 2015) or earlier, or a Mac mini (Late 2014) or earlier, you will also need this cable.

You can then follow the steps in this Apple Support article to setup your Time Machine backup: Back up your Mac with Time Machine - Apple Support. 

Jack

iCloud is NOT a backup mechanism for Macs.

Use Time Machine.