Bitcoin is free software and any developer can contribute to the project.
The first: Code Review

Bitcoin Core is security software that helps protect assets worth billions of dollars, so every code change needs to be reviewed by experienced developers

The second: Starter Projects
Fix existing issues:  Before starting to write any patches for issues you find, you may want to comment on the issue to make sure nobody else is already working on it.
Write tests
Documentation: you should start by exploring the developer documentation.
And finally, Developer communities : you can read their rules of conduct which host discussions about Bitcoin development as
IRC Channel #bitcoin-core-dev on freenode.
Bitcoin Core Slack Channel
Bitcoin StackExchange
BitcoinTalk Development & Technical Discussion Forum
 Good luck for you