Deadlines to register are as follows:
Based on today's statutory requirements, you must be registered at least 7 days before a general November Election (or if registering by mail, the mail must be postmarked by that 14th day). You may register from 9:00 AM to 12:00 Noon the day before any election if you turn 18, become a US citizen, or have just moved into the town after the 7 day deadline.
If you wish to vote in a Primary Election, your application must be postmarked by the 5th day before a Primary (OR received by your Registrar of Voters or a voter registration agency by the 5th day before a primary).
You may apply in person to a Registrar of Voters Office until 12:00 Noon on the last business day before a Primary.
If you live at school part of the year, you may register in either that town or in your hometown, but only one.
You can get all the registration forms you need here.