Create different encryption algorithms in any language. If there isn't a folder for your language, create one. e.g.:
Create new file -> cipheringMethods/Java/myAlgorithm.java
Contribute with a valid algorithm, and participate in Hacktoberfest.
You can also improve an existing algorithm (providing a decryption method, creating a cli or gui...) It's up to you, but make substancial changes.
That's it. Happy hacking!