For example the first day is typically when you see your work area learn your specific day-to-day workflow and schedule meet your coworkers get familiar with different areas of your office or building and much more. Depending on how much time you have, you can either spend that day just getting a feel for the company and getting a feel for the people you'll be working with or you can spend it doing some "homework" of your own.
For example, if you're going to be working with a lot of new people, you might want to talk to them on the phone or in person and learn a little bit about them. You might also want to do some research on the company and the people who work there, learn about the products or services you'll be working with, and understand how the company works.