The extension (Form 4868) is a form that you can file if you owe money and cannot mail your documents by the deadline which is usually April 15th. Please note that the extension is filed only to extend your time to file. If you owe money to the tax office, you would have to pay the liability by the deadline still. Also, you cannot file an extension after the deadline. The extension is not needed (on Federal level at least) if you have a refund or zero balance (you don't owe nor expect a refund).
You can file an extension through the IRS/state tax office's official website, however, keep in mind that a tax identification number (SSN or ITIN) is required.
If you’re a nonresident or resident alien and you don’t have and aren’t eligible to get an SSN, you must apply for an ITIN. Although an ITIN isn’t required to file Form 4868, you’ll need one to file your income tax return. For details on how to apply for an ITIN, see Form W-7 and its instructions. If you already have an ITIN, enter it wherever an SSN is requested. If you don’t have an ITIN, enter “ITIN TO BE REQUESTED” wherever an SSN is requested (https://www.irs.gov/pub/irs-pdf/f4868.pdf).