Some of the things:

1. Desperately try to not forget everyone's name.
2. Forget everyone's name.
3. Set up workspace.
4. Explore the product.
5. Read the style guide.

I think the first few weeks the most important thing involves learning the company, people, business processes, etc...

Yeah, I could say "set up the computer and software" etc... But that's sort of a given... I think the more important thing has to do with the people and the business itself. Learning how the departments communicate, learning about how people relate to each other, etc.... Is a lot more important than the technical aspects... Every one of us can do the technical stuff alone in our bedrooms but a big part of working professionally at a company is the communication, business rules, and stuff like that.

Besides all the typical HR or social stuff which is probably same for any job, these below are more specific for developers:

1. 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?
2. Understand the project management process, how are features/tasks assigned, what is the release process, how are tasks tracked, how bugs are tracked.
3. 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.
4. 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.