Besides all the typical HR or social stuff which is probably same for any job, these below are more specific for developers:
- Understand the product (although it might be good to do so before you even started the job), and actually use the product. Also what are the KPIs of the product?
- Understand the project management process, how are features/tasks assigned, what is the release process, how are tasks tracked, how bugs are tracked.
- Understand the dev environment, and set it up: how to build the product from source code, how to run locally, how to unit test, how to run in test environment, how to commit your changes.
- Understand the high-level technical architecture of the product, and (if you are assigned to a specific area, since if you are part of a big project, usually you'll assigned an area to specialized in.), how your assigned area fits into the overall architecture.