You will have to log in with the same username and password that you used to file. If you have forgotten your login credentials, read the TurboTax faq- forgot username and password
If you had multiple accounts, click here
If you didn't use TurboTax, you can
also get free transcripts of your federal return from the IRS- http://www.irs.gov/Individuals/Get-Transcript
You can read the TurboTax Faq on how to get a copy of your tax return for an earlier year:
https://ttlc.intuit.com/replies/3301525