Once you're eligible for Medicare, you'll have an Initial Enrollment Period to sign up. The Initial Enrollment Period starts 3 months before your 65th birthday and ends 3 months after. Make sure to terminate your Get Covered Illinois coverage as soon as you enroll in Medicare.
If you are enrolled in Medicare Part A or Part C, or are eligible for premium-free Medicare Part A, you are considered to have another source of comprehensive coverage and will not be eligible for premium tax credits through Get Covered Illinois. In most cases, it's to your advantage to sign up for Medicare when you're first eligible. If you decide to keep your coverage through Get Covered Illinois, you will have to pay full price.
Visit Medicare.gov for more information about this program, including information about how to enroll.