If you have filed with TurboTax before its really easy
https://ttlc.intuit.com/questions/1900748-how-do-i-get-a-copy-of-a-return-i-filed-in-turbotax-online
You can obtain a transcript from the IRS website
https://www.irs.gov/newsroom/how-to-request-a-transcript-or-copy-of-a-prior-years-tax-return
I need a copy of my 2013 returns
You will need to contact the IRS for a transcript. You can only download three years or prior taxes in TurboTax.
Requesting copies of prior returns from the IRS