My first PR’s were mostly typo fixes in the documentation. There’s still plenty of those I think.
It’s probably easier to fix issues in the newer frameworks (ActionText, ActiveStorage), as these are smaller and less complex than for example ActiveRecord.
There is a “good first issue” tag but there’s currently only a 6 year old issue tagged
What I sometimes do is look at the issues with the least comments. I’ll see if I can reproduce the issue, try to find similar issues, maybe look at the git history. Then I’ll add a comment with any interesting findings in the issue. Maybe I won’t find a fix, but it could help another contributor.
Another thing you can do is look at the PR’s that get merged. What does the change look like, what does the commit message look like, what changes were requested. This helps in getting a feel for improving your own PR’s, what coding standards are used and what else is required for a successful PR.
I’d also recommend looking at the “Contributing to Rails” guide, which has some great tips: