Greens need a good wash to get rid of dust and grit: put them in a big bowl of cold water and swish around a few times. Leave to sit for a couple of minutes to allow the dirt to sink to the bottom of the bowl, and then lift the leaves out with your hands and drain in a colander.
You can eat true spinach stalks and all, unless the stalks are very big, in which case give them a trim. Perpetual spinach is tougher. The ribs are best snipped or torn out and discarded (unlike chard, they are not particularly good to eat).