Where should I look or what criteria should I consider when seeking places to contribute code?
This helps ensure you understand the issue, provides motivation to work on it, and ideally to continue to support your work into the future.
I find there's no shortage of such opportunities, and that the interaction with the project owners/maintainers has helped me to learn productive communication strategies, good approaches to proposing new ideas, etc.
As the sibling comment says, find a project you use and like. Start with something tiny, like fixing a typo or implementing a small feature. Try to implement it in only 2 or 3 days, so if the maitainer doesn't like it or is a moron you have not wasted too much time.
Each project has it's own weird rules, like space vs tabs, or how much tests to add, or style in the name of the comits, or rebase vs merge or ... So expect to make a few corrections in your first PR until you learn the weird rules.
Sending good bugs reports is also a vvery important contribution. It's good to see haw they solved them to understand the structure of the codebase and weird rules, becuse next time you can try to fix it yourself and send a PR. But remember that a good bug report is useful too.
fork, create a pr, done.