As a U.S. Citizen you are required to file a tax return each year reporting all of your world wide income from all sources. For that of course, make sure you're using the U.S. version of TurboTax, and not the Canadian version you use for filing your Canadian taxes.
The current online version of TurboTax can only be used to file a 2017 return. So for 2015 and 2016 you will need to purchase, download and installed the U.S. version for each tax year (2015 and 2016) on your computer and use that to complete those two years of tax returns. Always start with the oldest year first. That way, when you start your 2016 tax return, a lot of your recurring personal data will be automatically transferred into the 2016 tax file, from your 2015 tax file, saving you time from having to enter duplicate information a 2nd time.
You can purchase TurboTax 2015 and 2016 online at https://turbotax.intuit.com/personal-taxes/past-years-products/ After you pay for it online, you can download the installation file and install it on your computer. Just make sure you are under the correct tab for the year you desire to purchase. Otherwise, if you purchase the wrong year, you own it.
Note that the link I provided you above is for the U.S. versions of TurboTax 2014, 2015 and 2016. TurboTax only supports their software for the current tax filing year and three years back. So at this time TurboTax is no longer available for tax years prior to 2014.
If you have any further questions or concerns, then please by all means, post back. If I can't help or answer your question, then I'll find someone who can.
