If you need a little more time to get last year’s financial affairs in order, you’ll need to file for an extension.
For an automatic six-month extension, use IRS Form 4868, which will require you to estimate your tax liability, based on the data available to you. (You won’t have to make a payment immediately, but you will owe interest on your tax bill.)
Just don’t wait until the last minute. Last year, the online service went offline due, in part, to a flood of traffic.