If you purchased health insurance from the Health Insurance Marketplace (ACA) you should have received a 1095-A from your insurance provider. If you are using Turbotax, they will ask you for that information if you go through the "guide me" step-by-step process. Or you can enter 1095-A in the search field and click on the "Jump to" link in Turbotax. If you didn't pay anything, you should still have received a 1095-A showing the enrollment premiums and the monthly advance payment of premium tax credit.
Enter $0 (or leave it blank) under medical insurance premiums since you did not pay anything out of pocket for your health insurance. If your insurance was procured through a state marketplace or Healthcare.gov, you will receive Form 1095-A. That form must be entered into TurboTax. If you had different insurance (not through a state marketplace or healthcare.gov), you may receive a 1095-B or 1095-C, but you won't receive the 1095-A form and you can skip that section.