As a F-1 visa holder, you are considered as "exempt" meaning not counting days for five years starting from the date of your first arrival in the United States. After the five years period, you will start counting days by using the Substantial Presence Test to see if you qualify to be treated as a US resident for tax purposes.
In your situation, you are considered as a nonresident from 2014-2018 filing a Form 1040NR. Starting from 01/01/2019, you will start counting your days. If you meet the Substantial Presence Test in 2019, you will be considered a US resident for tax purposes and file a Form 1040 for your tax year of 2019.
For more examples, see :https://www.irs.gov/individuals/international-taxpayers/alien-residency-examples
-As TurboTax does not support Form 1040NR, you will not be able to use TurboTax to file your nonresident taxes. However, we have a partnership with Sprintax who offers filing solution to our nonresident TurboTax customers. See Visit the TurboTax/Sprintax site.