Per IRS, you cannot file a Form 1040 or the required statement until you meet the substantial presence test in 2018. ( See attached link below) If you have not met the test for as of April 15 of 2018, you can request an extension of time for filing your Form 1040 until a reasonable period after you have met that test. To request an extension to file until October 15 of 2018, use Form 4868, Application for Automatic Extension of Time to File U.S. Individual Income Tax Return.
Please note that once you make the first-year choice, you may not revoke it without the approval of the Internal Revenue Service.
For more information, please see
https://www.irs.gov/individuals/international-taxpayers/residency-starting-and-ending-dates
last two paragraphs under Residency Starting Date under the First-Year Choice
