Medicare eligibility begins after you have received 24 months of Social Security disability benefits. Note that to receive Part B of Medicare (which pays for doctor visits), you pay a premium that will be deducted from your Social Security disability monthly check.
Disabled people with relatively low income and assets may be eligible for other programs that pay for medical expenses not covered by Medicare and/or pay the Medicare premium for you. To find out if you are eligible for any such programs, you need to check with your county welfare department.
If you have health insurance coverage already, you need to figure out how Medicare works with your health insurance. Many health insurance policies state that Medicare is to provide the primary coverage with your present health insurance paying only for what Medicare doesn’t cover. You need to check with your health insurance company when you get your Medicare card.