I will say try to understand their code, architecture and read documentation if available. Setup your working system properly so that later you won't face much issue. Interact with your manager and teammates as much as possible(not when they are working on their project and don't bug them much). If they are using a new technology then try to do some mini project with the new technology in your spare time to get properly acquainted with that tech. If you find something can be improved then don't hesitate to bring it to the attention of your team. If possible try to solve some low priority bugs to get more comfortable with the code. Extending the sentence in my first line if you feel documentation is not available then document it and share with your team.