You can not use turbotax to complete and file ONLY a state return. You MUST first complete the federao return. Then, and only then can you purchase the state module. The state module installs as an add-on to the federal program. Without completing your federal return first (weather you file the federal return or not) you can't even start your state return with any assurance it will be accurate.

i filed my 2015 federal with turbo tax...how do I do my 2015 state taxes?

DId you use the online version of TT 2015 back then? Or the CD version that you installed on your computer?

On line version

First, you need to log in to the same exact account you used to file the 2015 federal return with, and download the .tax2015 file. Then you need to purchase and install TurboTax 2015 on your computer. After intstallation, start a mock federal return so that you can then purchase the state program. (Do that under the STATE tab after you have started the mock federal return.)
After you have purchased, downloaded and installed the state module, close the proram and delete the .tax2015 file in the documents/turbotax directory. THen place the .tax2015 file you downloaded into the documents/turbotax directory. WHen you start the program now, you'll be able to open your already filed federal return, go to the state tab and start your state return.
You can purchase TurboTax 2015 at <a rel="nofollow" target="_blank" href="https://turbotax.intuit.com/personal-taxes/past-years-products/">https://turbotax.intuit.com/persona...>
MAKE SURE you are under the correct tab. Otherwise, you will purchase the wrong tax year, and if that happens, you own it.
Also be aware that you can not e-file a prior year return. So you will have to print it, sign it, and mail it to the state. When you print it (by selecting the "print and file" option) an instruction sheet will be printed with it. READ THE INSTRUCTIONS. There will be two sets of instructions. One for the federal return (which you don't need) and the other for the state return.