Hello,
0) learn to use Google. Getting the following two links took about 12 seconds
1) if you want to contribute to Open Source in general, go to [1]
2) if you want to contribute to Django specifically, go to [1], then go to [2]
3) profit!
Best,
Gergely
[1] http://developer.com/open/how-to-start-contributing-to-open-source.html
[2] https://docs.djangoproject.com/en/1.8/internals/contributing/