If you aren't eligible for premium-free Part A, and you don't buy it when you're first eligible, your monthly premium may go up 10%. You'll have to pay the higher premium for twice the number of years you could have had Part A, but didn't sign up.
If you were eligible for Part A for 2 years but didn't sign up, you'll have to pay the higher premium for 4 years. Usually, you don't have to pay a penalty if you meet certain conditions that allow you to sign up for Part A during a special enrollment period.
If you have limited income and resources, your state may help you pay for Part A, and/or Part B. You may also qualify for Extra Help to pay for your Medicare prescription drug coverage.