If you want an automatic six-month extension to file your tax return, you can file Form 4868, Application for Automatic Extension of Time To File U.S. Individual Income Tax Return, by April 15. However, you must pay any tax you estimate will be due by April 15 or you may be subject to interest and penalties on overdue taxes. Your actual return would then be due on October 15.
The file is in Adobe Portable Document Format (.pdf), and you will need the free Acrobat Reader to view and print the file.