Good news: You can have a six-month extension. Bad news: You still have to pay your taxes by April 15. File Form 4868 (download a copy from irs.gov or your tax prep program will provide one), and use last year's return to estimate what you owe or let your tax software do it for you. It's better to overestimate and get a refund later; if you're under by more than 10%, you'll owe interest of 7% on the amount you underpaid by, plus a penalty of up to 25% of the underpayment.
NEXT: I can't pay what I owe. What can I do?