Contribute for the first time to an existing project
Contribute to an open source project that you like or may want to strongly depend upon in the future.
Many projects have a good first issue label, which is a good place to start. You can also look for issues that are labeled help wanted or easy.
There's also the Rust Mentorship program, which pairs up new contributors with experienced Rustaceans. You can find more information about it at: https://rustbeginners.github.io/awesome-rust-mentors/
In The weekly Rust Newsletter you can find a list of projects that are looking for contributors.
If you are a seasoned programmer already you might also opt to already start making your own new project or porting a scoped project from another language to Rust.