You will register for classes with your departmental advisor. Be sure to contact your department's academic administrator before you arrive so that s/he can give you the name of your advisor and inform you of the registration process.
Registration Day for upperclassmen (including transfer students) is Tuesday, September 3, 2013.
When you meet with your advisor to register, s/he will have access to you online registration form and will be able to advise you appropriately in class selection.
Make a note of which classes you have scheduled and start attending those classes immediately. You will be able to check your status of registration on WebSIS starting the week after Registration Day.
Complete information on registration processes, can be found on the Registrar's site at: http://web.mit.edu/registrar/reg/.
See the Class of 2017 site for more information about the Science Core and other GIRs.