If you are that far behind in preparing and filing your tax returns, this may not be a good "do it yourself" project. You should seek some local paid professional tax help.
As TurboTax JulieR says, software is no longer available for tax years earlier than 2015--and the deadline for 2015 is next week on April 15, 2019 to get a refund for a 2015 tax return or it will be too late.
You cannot get tax refunds for any years earlier than 2015--so no refunds for 2011, 2012, 2013, or 2014. If you owe tax the IRS still wants you to pay.
If you decide to take on preparing the returns yourself--each tax year should be mailed in its own separate envelope. Do not combine them in the same envelope or they could be mis-handled or overlooked. It is not possible to e-file any year except a 2018 return.
When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s. Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS and/or your state received the return.
Remember to prepare your state returns as well—if you live in a state that has a state income tax. You can get state forms from the state's web site: