From the IRS:
It’s a good idea to have all your tax documents together before preparing your 2012 tax return. You will need your W-2, Wage and Tax Statement, which employers should send by the end of January. Give it two weeks to arrive by mail.
If you have not received your W-2, follow these three steps:
1. Contact your employer first. Ask your employer – or former employer – to send your W-2 if it has not already been sent. Make sure your employer has your correct address.
2. Contact the IRS. After February 14, you may call the IRS at 800-829-1040 if you have not yet received your W-2. Be prepared to provide your name, address, Social Security number and phone number. You should also have the following information when you call:
• Your employer’s name, address and phone number;
• Your employment dates; and
• An estimate of your wages and federal income tax withheld in 2012, based upon your final pay stub or leave-and-earnings statement, if available.
3. File your return on time. You should still file your tax return on or before April 15, 2013, even if you have not yet received your W-2. File Form 4852, Substitute for Form W-2, Wage and Tax Statement, in place of the W-2. Use the form to estimate your income and withholding taxes as accurately as possible. The IRS may delay processing your return while it verifies your information.
If you need more time to file you can get a six-month extension of time. File Form 4868, Application for Automatic Extension of Time to File US Individual Income Tax Return. If you are requesting an extension, you must file this form on or before April 15, 2013.
If you receive the missing W-2 after filing your tax return and the information on the W-2 is different from what you reported using Form 4852, then you must correct your tax return. File Form 1040X, Amended U.S. Individual Income Tax Return to amend your tax return.
Forms and instructions are available at IRS.gov or by calling 800-TAX-FORM (800-829-3676).