A security camera hangs near a corner of the U.S. Internal Revenue Service (IRS) building in Washington May 27, 2015. Reuters

The Internal Revenue Service's tax season began Monday, meaning U.S. taxpayers can start turning in their electronic tax returns this week. Eager to file early but still waiting on your W-2? Here's everything you need to know about the federal deadline and when and how you have to pay Uncle Sam.

First, the good news: Your employer must get you a W-2 by the end of the month, so there is no reason to wait until the last-minute to file your taxes. More than 153 million individual tax returns will be filed this year and roughly four out of five will be prepared online using tax return preparation software. But you need the right information to get started.

"Taxpayers who are changing tax software products this filing season will need their adjusted gross income from their 2015 tax return in order to file electronically," the IRS states on its website. "The electronic filing PIN is no longer an option."

Wanting for cash back? Many people can expect to receive their refunds within 21 days. Roughly 111 million refunds were issued in 2016 for an average refund of $2,860. That was a 2.3 percent increase from 2015.

“For this tax season, it’s more important than ever for taxpayers to plan ahead,” IRS Commissioner John Koskinen said in a statement. “People should make sure they have their year-end tax statements in hand."

The deadline to file is April 18. The usual April 15 deadline was pushed back since it falls on a Saturday this year.

Need more time? You can always ask for an extension, but it will cost you. "Getting a filing extension avoids the late filing penalty, but it doesn't avoid the late payment penalty," Barbara Weltman, a consultant and author of books on taxes, law and finance, told the Associated Press.

Anyone who didn't have health insurance in 2016 can expect to pay a fine under former President Barack Obama's Affordable Care Act. The penalty is $695 for each adult and $347.50 for children. About 6.5 million taxpayers without health insurance paid penalties in 2015.