Your premium is higher if you don't sign up when you are first eligible - unless you're still covered by an employer plan.
You have to pick Part D plan
to cover Rx drugs. you'll also need a Medigap (Medicare Supplement) to pay the 20% Medicare doesn't pay. There are 10 standardized policies the insurance companies can sell. I bought one thru AARP for about $145 a month. It will pay for most of my expenses after the $147 annual Part B deductible. There are $20 office visit co-pays. There are cheaper policies. but you pay more out of pocket if you need medical are (which I do).
There or publications from Medicare.gov to help you chose a Medigap plan.
For low income seniors, there is something called Medicare Savings Program which I think is Medicaid since it is administered by the State. It pays your Part B premium and deductible so you don't need a Medigap policy. There's info on Medicare.gov about it.
Good luck ! I did this for a living (benefits ad ministration) and I struggled with researching and selecting a plan. This is why I am so opposed to giving a voucher to a senior and having them shop for their coverage.
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