The TurboTax program has to be purchased for each tax year. You need to complete and file the 2017 return first.
Note that you can not e-file a prior year's return. The IRS says so. Additionally, you can not use the online version of TurboTax for doing a prior year's return. The online version is for the current tax year only.
You can purchased the CD version of TurboTax 2017 at https://turbotax.intuit.com/personal-taxes/past-years-products/
Now make *SURE* you are under the correct tab for the tax year you need. Then make equally sure you get the right one for your operating system, be it Windows or MAC.
If you need help importing 2017 to the 2018 return, we can deal with that here after you complete the 2017 tax return and get it signed by your parents, dated and mailed off. When you mail it, send it return receipt required. Otherwise, if the IRS claims they didn't receive it and you can't prove otherwise, you lose.
