Take your time, learning a new code base takes lots of time. Get familiar with the language the code is written in, and the style used to write that code, if you aren't already familiar with it. Learn about the code before learning the code, with large code bases its nearly impossible to remember exactly what every piece of code does, and its pointless because the code is always there to show you what it does, if you can find it, so learn how to find the code you want.
You have to start small, and your employer will probably help with this by only assigning small tasks at first that only require localized changes and gradually work towards bigger changes. Also never be afraid to ask your team members questions, especially early on you need to build relationships with them just as much as you need to learn the code.