Feel free to work on any of the issues, or open an issue yourself if you have an improvement or you found a bug.
If you want to be sure that no one else is working on the same issue, go ahead and leave a comment that you're working on it. But if you end up not working on it anymore, please leave another comment that the issue is open again.
If you've worked on an isssue, go ahead and open a pull request!
Lastly, your help is very much appreciated ^^.
If you're new to contributing on GitHub, you can read all about contributing here.