I recently was asked in a comment if I'd have some advice for someone who is going to start her PhD in September.
Here's a list of 5 important first steps:
1. The paperwork
There might be a whole set of different institutions that require you to fill out paperwork,and even more if you are an international student. After arranging the paperwork as required by my department, I thought I was done. Only much later on, I understood that I also had to register at the immigration services and that I had to fill out documents for the office of the beadle. In fact, I only finished my "initial" paperwork at the moment when I was 2 years and 5 months into my program. I would have preferred to have everything out of the way as soon as possible (also because I dislike administrative tasks very much).
Get to know your library: both the online library system and the library building. Figure out what you need to do to get library access, and learn how to search for papers and documents in the system. During my studies at Georgia Tech, I had learned from other graduate students how to find papers, and that knowledge turned out to be a life-saver when getting started in Delft.
3. Get to know the people in your research group
Take some time to get to know your new colleagues - their field of expertise, but also their skills so you can learn from them and ask for advice. When I arrived to Delft, I took a flying start by starting my experiments within the first 2 months - but that left me with less time than I would have liked to have 1-on-1 conversations with everyone in my research group.
4. Understand the requirements
Do you need to pass qualifying exams? Do you need a certain amount of credits in coursework to graduate? Do yo need to publish a certain amount of papers? Just make sure you have all of this information very clear from the beginning, so you can plan accordingly and avoid unpleasant surprises. In my case, I know that all that really counts towards my degree, is the thesis and propositions.
5. Explore your surroundings
Get acquainted with your new city and campus, and put important phone numbers in your phone (campus security, housing office if you have on campus housing, cab company, local police). If you'll be living in a certain area for 3 or 4 years, it might be worth getting to know it well from the very beginning.