How to restore marble and granite surfaces?
Assuming you don't have extensive scratching or damage to your countertops, sometimes a simple acrylic sealer/refinsiher will work great.  You will need to clean the tops thoroughly with a non abrasive cleaner and rinse completely.  After they are dry, simply apply the acrylic sealer with a lint free smooth cloth evenly over the whole surface.  Let this dry for an hour or two and repeat the application. After the last coat is completely dry, buff with a soft cloth.  Usually two or three very thin coats and a good buff will restore a nice shine. This should last a year or more depending on the amount of use the tops get.  Do not use abrasive cleaners in the future. Simple glass or all purpose cleaner is fine. You may also consider having some anti-bacterial solution (looks like water) in a spray bottle for quick safe cleanups.