If you are a new employee, you need to fill out the IRS W-4 Form when you are hired so that MIT can withhold the correct federal income tax from your pay. You can download the form from the link above.
The IRS suggests you review your federal tax withholding amounts annually. It provides a Tax Withholding Estimator and related information on its website.
The IRS and VPF HR/Payroll recommend you review your withholdings whenever you have a major life change (marriage, divorce, birth or adoption of a child, new job, significant changes in income or other changes to your personal finances), or if after filing your taxes each year, you want to adjust (for either over withholding or under withholding).
If you need to make changes, you can download the same form and resubmit it by mail to: 
MIT Office of the Vice President for Finance
77 Massachusetts Avenue
Building NE49-4000
Cambridge, MA 02139 
or by fax to: 
617-258-9700