I have gained some experience during my open source journey and I am passing it on. It worked for me and I hope it works for you too:

At first understand how to use the package you want to contribute to. Then pick up an issue with a somewhat clear goal of what you have to achieve to solve it (doesn't matter if the issue is big or small). Then start hacking with the code. Change some parts of the code and see how it affects the results. Within an hour you will definitely have a better idea of how the code works (only the part that you are working on). Now you can make few changes to it and open a WIP PR. In this way, you are contributing to the project even if your PR get closed (since your closed PR will help future contributors to understand the issue better).

If you get stuck somewhere, ask the maintainers about what you should do next. Maintainers are very supportive to new contributors and sometimes they end up sharing the whole code with explanation for completing your PR. After a few PRs you will have enough knowledge to smoothly work on the project.

Happy Coding :)