Ah, what a perfect opportunity for me to answer this, having started a new job just a week back.
The best way is obviously to dive in to code. Pick out a part of the application that you think you understand the most and trace the code by inserting logs (or breakpoints) to get a grip of the coding pattern. Then, build a similar feature or a solve a bug in the part of the code that you understood. Once your first PR gets merged, you will be touching all parts of the code very very soon.
Happy coding! I am still in the process of shipping out my first feature, I'll keep you posted on how it works out for me. :)