If your mother is on a routine set of medications, go to
www.medicare.gov and use their Medicare Prescription Drug Plan Finder. You will need to enter her medications into a list, one by one, adjust the quantities to what she actually takes, and then submit. A list of all the drug plans for her area/zip code will be created, showing how many (11/13, 7/13, etc.) of her drugs are covered by that plan. Then you need to look at the specifics of the "coverage" to see if they provide the coverage she needs. Some may be "covered" but require authorization,m some may be "covered" but only after going through a step program (try cheaper drugs first), etc.
Then go to
www.medicaredrugplans.com or
www.medicaredrugplans.com/ratings, input the zipcode, and all the plans in the area, with premiums, types of coverage, etc., will be listed, This is an excellent site to compare costs, not just monthly premiums but deductibles, coverage, the "under $20 rule", etc.
If your mom just needs general coverage, and is not on any specific medication, just go straight to medicaredrugplan.com. There is a comparison of the "top 100 drugs", but in someways this is just a loss leader, like supermarkets have, that make plans appear to provide better coverage than they do, because they provide coverage of these 100 meds, but not necessarily anything else.
If you have questions, post them here, there are quite a few who can try to help answer them.