Are you using the Online version? Online is only good for one return per account. But you can use the same email address for 5 accounts. So don't use your account or you will overwrite your return.
Sign in or start a new account here,
https://turbotax.intuit.com/personal-taxes/online
The Desktop program can do unlimited returns and efile 5 federal for free. You can buy the Desktop CD or the Download program here,
https://turbotax.intuit.com/personal-taxes/cd-download
If you are using TurboTax Desktop, you can start a new return by clicking on "File" at the top left of your screen and then select "New Tax Return". You can also click on the blue button that says "Start a New Return" on the screen when you open your program.
As VolvoGirl mentioned, if you are using TurboTax Online you can only complete one return. But you can use the same email address for up to five accounts.
In TurboTax Desktop, you can select "File" at the top right of your screen and select "Start New Return"
Or you can click on the blue button "Start a New Return" when your program opens.
Click here for How Do I Clear and Start Over?