Tupperware is a brand of plastic food containers with lids that seal very tightly, so what is inside stays fresh and does not leak in your bag. But the useful thing for a learner is what has happened to the word: people say it for any container of that kind, whatever brand it really is. 'Put the leftovers in a Tupperware', 'bring your own Tupperware' — nobody checks the label. That is called a brand name becoming the general word, the same thing that happened with Post-it or Xerox. Because of that history, you see it both ways. Strictly it is a brand, so capital T and no plural, like a material: 'a piece of Tupperware', 'a Tupperware box', 'all my Tupperware'. In everyday speech people happily say 'a Tupperware' and 'two Tupperwares' and write it with a small t. Neither will make you sound wrong; the capital-T, no-plural version is safer in writing. One more piece of the picture: for decades the company sold at home parties, where a host invited friends over and someone demonstrated the containers — 'a Tupperware party' is a phrase you may meet in films.