In the first week everything starts at once: classes, the dormitory, a new city. These three things are best done within the first two weeks.
ARC
You book an immigration office appointment online. The card must be applied for within 90 days of arrival, but queues are long, so book in your first week.
Phone number
Until you have an ARC, you get a prepaid number with your passport. Once the ARC is issued, you can switch to a cheaper monthly plan.
Bank card
Many banks will not issue a card without an ARC, so for the first month you live on cash and an international card. Plan for that in advance.