Once you turn 65 you are automatically eligible for Medicare coverage. That provides a base level of health insurance that will take care of a lot of your medical expenses, but probably not all. You will be required to pay a premium for some of your Medicare coverage, and you will probably want to purchase a private Medigap policy to cover all the costs that Medicare doesn't.
Before age 65, if you don’t have retiree health benefits from a former employer, you’ll need to purchase an individual plan, either from a broker or via an online exchange. You may also be eligible for COBRA benefits for a short period after you leave your job.