Fresh spinach shrinks enormously as it cooks: a big bag only gives a small panful. Baby leaves are eaten raw in a salad, the big leaves get cooked. Frozen spinach is very good and cooks without thawing.
Firm, dark green leaves, with no yellow or wet parts.
Remove the thick stems. Wash the leaves twice in plenty of water, the sand clings, then spin-dry.
Three minutes, washing included.
In the fridge, two or three days, it spoils fast. Cooked, two days.
The page "Sautéed vegetables, in a pan or a wok": spinach wilted with garlic, two minutes.