Go to this IRS website for the Form 4868, complete print and mail to the IRS - https://www.irs.gov/pub/irs-pdf/f4868.pdf

Note - The extension gives you an extra 6 months to file the tax return, not an extension of time to pay any taxes owed.  Taxes owed are due to be paid on or before April 17, 2018 to avoid a failure to pay penalty.