The biggest things that help me get started with any large codebase are this: First, use the software. What does it do? Learn it, learn what the buttons do, read the user docs, try to understand as much as you have time for. You can never hope to reason about the code behind something, if you don't understand what the code is trying to accomplish. From there, pick something to familiarize yourself with just a small portion of the codebase. This can be something from the issues or bugs list, or it could be some new feature that you want to or are told to add, or it could be something as simple as trying to figure out what the "correct" way would be to change the color of a button or background of a form. Be ready to throw away your work and start over multiple times as you learn the caveats of the codebase and read the other developers' code.