If you purchased health insurance from the Marketplace, the Marketplace sends you a Form 1095-A in the mail in January. You enter the Form 1095-A in the Health Insurance section in TurboTax. When you tell TurboTax that you had Marketplace insurance ("Obamacare") it will prompt you to enter the Form 1095-A.
If you had Marketplace insurance and did not receive Form 1095-A, call the Marketplace and ask them to send you another copy. Make sure they have your correct current address.