[Django]-Where do I set environment variables on my Django Digital Ocean server?

You can create environmental variables inside your .bashrc file in your home folder.
Just open the .bashrc file from home folder

sudo vi ~/.bashrc

And then at the end of the file, add your variable

export SECRET_KEY='your secret key'

then save it, and try running source command on the file so as to enable the variable(So that it gets applied without restarting the system)

source ~/.bashrc