Step one, show up on time and as ready to go on day one. If you are anal like me, you will have already driven the route, on a regular work day, to make sure you know what you are doing. It is perfectly acceptable (and smart) to ask them about where to park (including the cost, if any), particular roads or routes to avoid, etc.
Feel free to get in touch with your POC and ask for reading material ahead of time. Ask them what areas they may need help with on the work you'll be doing. Ideally, you should have learned about this as part of the interview process. But you may have a new POC, such as your new boss, who can give you the real information.
As part of the interview process you also should have researched the company, its industry and its competitors. Time to review those notes and make sure it is all fresh in your mind. If you will be working for, or with, a government client then I highly recommend reading every available Government Accountability Office (GAO) report you can get your hands on. The Congressional Research Service (CRS) also has a lot of good material in issues, industries and particular departments/agenies. Harder to find, but good as well, are Inspector General (IG) reports. Many of those are not release for public consumption.
Come in and listen well. Ask questions, but not too many. Too many is defined as the point in which people avoid talking to you, or start rolling their eyes when you talk.
Wait on all your brilliant recommendations, and when you offer them, start small. Offer them in a polite way, one that doesn't imply that they've been doing things wrong, or in an overly simplistic manner. And avoid the dreaded "When I worked at _____" syndrome. Wherever you were before wasn't perfect, if it was, you wouldn't have left. You need to strike a balance between being ready to go on day 1, but not being too eager.
And remember to wear pants and zip your fly. That's a day 1 no no.