how do i back up my computer to an external hard drive

Your mac came preinstalled with a backup utility called Time Machine. You can access it from the Dock or set it up from System Preferences. The instructions are in the article below. The drive should, preferably, be 2 - 3 times the size of of your internal drive.

Back up your Mac with Time Machine:

https://support.apple.com/en-us/HT201250