I would strongly recommend to directly start coding yourself and ask for help along the way. It’s totaly fine to not know things and to make mistakes at the beginning.
On the other hand, code reviews usually require a deeper understanding of the code since you only see a small part (the changes) and need to remember the rest to deduce what consequences a certain change has on other parts of the program. Nonetheless, if you really prefer this way to start contributing, then the following PRs are up for review: https://github.com/JabRef/jabref/pulls?q=is%3Apr+is%3Aopen+label%3Aready-for-review