When you are in, be prepared to ask questions, to understand everything before everyone gets busy in their own world. Since it is a new place and we understand a new environment can be very confusing, hence we advise you to sort out things before the serious stuff begins.
Start reading as much as possible because you will encounter technical papers and reports and it is mandatory that you are well acquainted with your field. Select the material that will benefit you more rather than trying to read everything and retaining nothing, because there is no visible end to the material that is available.

There will be many professors who will have varied inclinations and interests, and hence before formulating an opinion talk to other students. Also, there are many seminars which are held from time-to-time which is attended by students and professors alike. Attend these seminars and then be on the look-out for an exclusive interaction session with different professors. Make yourself visible as this is bound to be useful in the future where you may need their guidance and assistance for projects and other references.  

Brace yourself to be drivers of your own course. Remember under-graduate programs when everything was easily available (read spoon-fed). Expect no such favours here, and ambition and motivation will be the essential ingredients. This is because you will not just be a student, but also a teacher and supervisor in some cases. And hence be confident about yourself and your abilities because you may have and you should be on the look-out for opportunities to speak and voice your opinions. Hence, it might be beneficial if you take up different small courses along the way. This will help you stay in touch with the subject and also help you build different insights, which will be very helpful.