I've been there. You have to communicate with your manager and be honest about how much you can do how quickly. Start small, with one component in one programming language. Learn the language and the frameworks. Ideally you'll be allowed time to do so. After you're familiar with the technologies the code will be easier to understand. Make notes on the code. Read the docs and if they're missing info, find out and fill it in; that way you're learning and contributing. I like starting from the database models, data is usually the heart of an application. It's important not to feel like you have to rush. Learning big things takes time.