Your employer was required to have your W-2 available to you by January 31st. Contact them for a copy.
You can file for an extension, it's free. You can start a return and use your end-of-year totals found on your last pay-stub to ESTIMATE your tax liability. If you will owe, you'd be safe to pay about 10% more as a cushion.
If you do pay and file an extension, be sure to report that payment when you finally do file for 2018.
