First: decide what your skills are, and where you can contribute.
Try the Tor Project first: https://www.torproject.org/getinvolved/volunteer.html.en
The community is great there and very accepting of newbies that have the skills.
Signal / open whisper systems also are open source and accept pull requests:
https://github.com/whispersystems/
Read the anonbib:
Also, there are other mixnet/darknet projects that could use help, such as i2p:
https://geti2p.net/en/get-involved
And if you want decentralized, mesh communications, cjdns and the Hyperboria experiment:
Don't forget: if you are a developer, you actually make money, too. Vote with your dollars as well - donate to these persons. Many are 501c3's and it is tax deductible for you.