Hacktoberfest 2017 (sign up on that page)! Hacktoberfest is open for all open-source projects like ours. After registering, if you make at least four Pull Requests to a public repo on Github during October (need not just be to Evennia), you win a limited-edition T-shirt!
The help Evennia out and get your T-Shirt, look at our Issue Tracker. I have marked some issues with "Hacktoberfest" but you could take on any issue you want. Take a look in particular at the Unit test issue if you are looking to get into contributing on a smaller scale while helping us tremendously.
If you have any questions on contributing (or it's your first time making a Pull Request), don't be shy to drop into #evennia on irc.freenode.net or ask in our forum/mailing list. Have fun!