For your regular, routine cleaning, the best cleaner is actually just a mild dish soap and some water. You can set up a bowl or a bucket, or even better, just wet your cleaning cloth and put a few drops of dish soap in the center of the cloth. You’ll want to make sure to use a non-abrasive cloth to prevent scratching the surface of the granite (something like a microfiber cloth is ideal).
After this, wring out the cloth, wet it with water, and wipe the counters again. Once you’ve washed it with this, wipe dry any excess water on the countertops. The other route is to purchase a commercial granite cleaner.