If you are using TurboTax Online, you cannot prepare and file a tax return for another person using an existing TurboTax account. You'll need to create a new account for to file for another person.
In TurboTax Online, you cannot work on more than one return at the same time. You have to log out of the first return, and log in the other account.
If you are using TurboTax Desktop, you can start a new return for yourself by clicking on Start a new return on the Welcome page of TurboTax.
Please read this TurboTax Help topic on how to start a new tax return for another person.