You must submit Form 4868 to file for a tax extension. This will give you an additional six months to file your tax return, making your new tax deadline October 15.
Your request for an extension must be made by the normal tax deadline which is April 15 for most years.
Please note: just because you have extra time to file your return does not grant you extra time to pay. An extension only provides you with a reprieve on the paperwork, not the payment if any is owed. On the same Form 4868 you use to request your extension, the IRS requires you to estimate the tax you owe and pay that amount. This tax payment will be adjusted in October by either a refund or an additional payment.
The good news, however, is that an extension allows you to file anytime between the normal April deadline and October 15 without the usual penalties and interest that accompany late filing.