this isn't really something i wish i had known, but i wish i had taken advantage of all that MAC has to offer. the university has so many things to offer us, but a lot of people don't take advantage of them. use csd if you have to, go to the student success center, talk to your ta's about your assignments, if you want to volunteer there's ample oportunities provided by CVA and other organizations. clubs are a good way to get involved too. having commitments at mac will probably make you feel more comfortable, more at home, and definitely a lot more grounded.
leave the mcmaster bubble. you are given a bus pass. use it
don't overwhelm yourself with trying to do your best, in the end balance is what will help you the most. make friends, have a social life, maintain relationships at home, and enjoy your first year!