In terms of things to do before you arrive, there isn’t that much. Make sure you get all of the paperwork done as soon as possible to ensure you receive your grant swiftly once you arrive, and be sure to apply for your maintenance loan on time, if you need one, so that it arrives fairly soon after your arrival in France.
It’s also worth considering the language course that, in Grenoble, runs during August. If you are not feeling confident with the language, it is a good way to ease yourself in before you start lectures. It is also a really good way to get to know people. I didn’t take the course myself but I have friends who did and they really enjoyed it. It’s run by CUEF, located on the university campus, which also run trips during the course. Information about CUEF can be found on this website http://cuef.u-grenoble3.fr/version-francaise/informations-pratiques/activites-culturelles/.
Before I arrived in France (and for the first few days) I was pretty stressed due to the lack of information from the university. There is a point of contact for the international students in the maths department, Odile Garrot, as well as a general International Relations office. Don’t worry about going or emailing and asking the really obvious questions (like when does term start!); most of the time they probably won’t know either but will make sure they find out for you.
Also, have a chat to anyone you know in the French department. Grenoble is one of the places that students in the French department can choose to study for their year abroad so it is worth asking around and possibly seeing if you can meet up before you go, or once you get here! I was put in touch with someone else studying in Grenoble by a mutual friend and in the first week here it was really comforting to have someone to call and go for a coffee with!