The first thing I try to do is to understand the directory structure, ie where should I be looking for files? Hopefully there should be a standard structure that's used. After that I'll typically try to dig in and fix a minor bug or two. This is especially helpful if you can narrow down the part of the codebase you're working on. I also recommend using an IDE like WebStorm which will give you the ability to jump to a function definition and will help you find the functions you're calling.

One thing I do NOT recommend is changing the code style, unless you're ready to take full ownership of the project. It can make it much harder for the project owner to merge in and if there are any lingering PRs those will typically need work to merge in properly.