There are 6 giant shopping centers in Prague - but there are hundreds of malls, shopping centers, and "passageways" for shopping in Prague. Almost every block has restaurants, shops, pubs, and a secret garden in the courtyard. Don't miss the food courts in the malls!
For grocery shopping, find a Marks & Spencer (the only grocer in Prague with English labels), Billa, Albert's, or Tesco (located in most metro stations and they all sell super cheap single bottles of beer). The DM Market or a Pharmacy has health supplies such as contact lenses (no prescription required), soap & toothpaste; organic foods, cosmetics, medicine, etc. There are also lots of Chinese markets with old food and fake cigarettes (bought yesterday from the grocer). Check out the many street markets for an interesting shopping experience. Remember: the closer you are to the city center - the higher the prices.