This is a long, involved process. Lots of things to take into consideration. You need to start with the Medicare site and find out what's in your area and what plan covers the prescriptions you need to take.
Medicare.gov - The Official U.S. Government Site for People with Medicare
Drug costs are only covered up to a dollar limit, then you have to pay for all of them up to another limit. (referred to as the "donut hole").
The plan won't substitute for what the doctor prescribes, but almost all plans use a formulary -- they only cover certain drugs. Others require step programs. You can only get the brand name if the generics are tried and don't work. You always have the option of paying full price for the brand name.
The Medicare site has a search function so you can input -- and save -- your drug list and then find plans in your area that cover those drugs.
Some plans are drugs only, others are like HMOs (Medicare advantage) and cover added doctor services and drugs.
It's complicated and frustrating. You need to find plans that don't cost more than the drugs you're trying to cover. I saved a lot of money by talking to my doctor about what generics I could use in place of the $150 a month drugs and also order some from Canada. With a lot of places like Target and Wal-Mart offering generics for $4 a prescription, it can save a lot of money.
I went through this last year and now I'm going through it again because my drug plan changed what they cover and how much it costs.
Best advice I can give is to use the Medicare site to get an idea of what plans are available and how much they cost and then check each plans' website for details.
And talk to your doctor and/or to the office adminsitrator that handles insurance stuff for his/her office. They're one of the best sources for information.