Tetragonia is not a member of the spinach family, but it tastes a lot like it. The leaves are slightly succulent - not quite as juicy as your Mom's jade plant, but still. As a result, when you buying it, you should make sure the leaves are bouncy/crispy without a hint of wilty softness.
To prepare it for cooking, strip the leaves from the stems and wash them well. Sauté like spinach; I like mine with lots of garlic and a splash of lemon or vinegar. Not that Tetragonia has quite a bit of oxalic acid in it (like spinach and chard do), so your teeth may get that sandy feeling after eating it. It's also why I don't like eating tetragonia raw: I find the oxalic acid sandiness is diminished when the vegetable is cooked. Also, because oxalic acid isn't super-great for your liver in general, don't go eating this stuff at every meal.