TurboTax Online can only prepare and e-file one federal return and up to five state returns per UserID. To file a return for someone else, you must create a new UserID for that person. You may create a new login (up to 5) tied to your same email address if you'd like.
If you are using TurboTax Desktop, first save what you're currently working on by clicking File > Save As. Then, to start a new tax file, click File > New Tax Return.
To create a new account for TurboTax Online, please see this TurboTax FAQ: