If you have a complex tax situation, then you should prepare 2015 first for carryover amounts. If your taxes are quite simple, then it doesn't matter as much. However, I personally recommend preparing them in order.
Your 2016 taxes can be prepared online. However, for 2015, you will need to download the prior year version of TurboTax.
Here is the link to get the 2015 software:
Prior
Year TurboTax Products
