It's that time of year again. No, I'm not talking about March Madness. I'm talking about taxes. The April 15 deadline is fast approaching and, if you're like most Americans, you haven't filed your taxes yet. But don't put it off too long. One common error many young filers make is missing the deadline.
"A big mistake is that ... millennials say, 'Oh, April 15—they don't really mean that'," said Annette Nellen, certified public accountant and professor of accounting and finance at San Jose State University. "No they really mean it. You can get extensions to file, but not to pay."
Even if you ask for and get an extension, you need to file a form requesting it by the deadline. And if you're granted an extension of time to file your return, that doesn't mean you have an extension of time to pay whatever you owe in taxes. You can still get hit with a failure-to-pay penalty. "Those deadlines are serious," said Nellen.
Here are other common and potentially costly errors people make, and how you can avoid them.
Read More Breaking bad... millennial money habits
Some of the most frequently made errors, tax preparers say, are the result of simple typos and mistakes in math calculations. One way to help prevent calculation errors is to file electronically.
"If you e-file, it is also a quicker way to get a refund and to get it processed," said Nellen, who sees everyone moving to electronic filing at some point. According to the Internal Revenue Service, at least 70 percent of taxpayers already prefer filing electronically over snail mail. You can use a tax e-filing product like Turbotax or, if you made less than $60,000 in 2014, you can use the IRS's free file program.
But it's still important to double check each field to make sure the information you entered is correct. At the least, minor errors can result in delays in getting a refund. But they can also up your chances of an audit or, in the worst case, cost you money in penalties.
Read More Stop making these mistakes on your tax return
When it comes to paying Uncle Sam, everyone wants to know what's deductible. One big deduction that many millennials can take: student loan interest. If you don't itemize your expenses and your income falls within certain limits, you can deduct up to $2,500 on the interest you paid on your student loans in 2014.
Other deductions are less obvious. If you relocated for a new job or if you were transferred to a new location and your employer didn't cover your moving costs, for example, those costs are deductible. Start shouting "show me the money!" because you can lower your taxable income by thousands of dollars if you deduct moving expenses like the cost of moving your furniture and traveling to the new location. Just note that in order to write off your move, your new workplace must be at least 50 miles farther from your old home and you must work full time.
If you are itemizing your deductions or your situation is complex—say, you have a couple different jobs, freelance, or live in one state but work in another—it may make sense to get an accountant to ensure you don't miss out on potential deductions, said Larry Luxenberg, a certified financial planner at Lexington Avenue Capital Management in New York.
Nellen also recommends reviewing the Pub 17 tax guide on the IRS website so that you don't miss out on money or set yourself up to get audited because of simple mistakes.
Read MoreWorried about an audit? No need, if you do this
Once you complete your filing, the fun part begins if you're owed a refund… free money! But experts say don't treat it that way. "With tax refunds, the average amount for millennials in the U.S. is about $3,000," said personal bankruptcy lawyer William Waldner of Midtown Bankruptcy in New York. "It's human nature for people to want to go out and have fun with this money."
But financial advisors caution against that. Luxenberg suggests the best way to use that refund is to treat it the same as a paycheck and use it to pay down debt or bulk up savings. "It's a matter of priority," he explained. "Build up your emergency fund, pay down high-cost debt, then start saving and investing."