The school year in the US begins in August. Once you’re in the University’s system, you will get emails telling you when classes start, how to enroll in classes, when orientation for new students start, how to set up your university email, etc.  This can be a lot to take on, especially if you are new to the university, to the city, and/or to the country. Remember that there is a departmental coordinator who you can email questions about this entire process. It's also a good idea to find a second year PhD student who doesn't mind helping you.