Did you look at the issue tracker? Choose the
"Easy Pickings" Tickets from there that will help you get familiar with django but before writing any actual code or documentation you must know what's going on in here if you don't then
freecodecamp django video will help you understand how django is used, moreover, go and look at the video
"Django in depth" by James Bennett which was made to help you understand how Django internally works. Once you complete all this then you would have enough knowledge to pick any "Ticket" and start contributing.
Thanks!