This week in rust usually has a section for projects that are asking for help. I think that's probably the best thing to get started.
That being said, rust has steep learning curve, so probably the best thing you can do is use it for your own projects so you get to know the language better before trying to contribute to other crates. That's not to say that it's not possible, but you'll be able to contribute high quality much easier if you already tamed the beast!
In practice, if you use rust, you'll find bugs, missing features, missing tests and documentation in your dependencies, and filing pull requests to fix those helps everybody, yourself in the first place.
Another possibility is taking on missing tasks from rosetta code, porting them to rust and preferably ask for review in the user forum rather than here, or on reddit, before publishing them.